Add compatibility with the sound theme and sound naming specs

Here we make the Oxygen sound theme compliant with the sound theme spec and sound naming spec:
http://0pointer.de/public/sound-theme-spec.html
http://0pointer.de/public/sound-naming-spec.html

Here I kept the original sounds in the folder the way they always were, that way, there won't be compatibility problems. I just added a new index.theme file as the spec requires and made relative symlinks with the new names in the stereo folder.

Mapping the sounds took a lot of guesswork since the original sounds were not made with the spec in mind. There are a lot of sounds that have no equivalent on the spec but I followed the naming rules when creating new names.

**System sounds:**
- Oxygen-Sys-App-Error - dialog-error
- Oxygen-Sys-App-Error-Serious - dialog-error-serious (not in the spec but it allows naming more specific sounds)
- Oxygen-Sys-App-Error-Serious-Very - dialog-error-veryserious (not in the spec. The name is composed of two words joined together because the "-" separates how specific the sound is, it's not meant as a simple way to separate words. In that case, if I added very-serious the code would always prefer this sound over the others.)
- Oxygen-Sys-App-Error-Critical - dialog-error-critical (not in the spec but it allows naming more specific sounds)
- Oxygen-Sys-App-Message - dialog-information
- Oxygen-Sys-Question - dialog-question
- Oxygen-Sys-Special - dialog-special (not in the spec. I'm also not sure in what situation this would be used, but I think starting with "dialog" should be fine)
- Oxygen-Sys-Trash-Emptied - trash-empty
- Oxygen-Sys-App-Positive - outcome-success and power-plug (not in the spec but it follows the same naming convention that will be used for the new Blue Ocean theme)
- Oxygen-Sys-App-negative - outcome-failure and power-unplug (not in the spec but it follows the same naming convention that will be used for the new Blue Ocean theme)
- Oxygen-Sys-Warning - dialog-warning
- Oxygen-Sys-Log-Out - desktop-logout
- Oxygen-Sys-Log-Out - service-logout
- Oxygen-Sys-Log-In - desktop-login and theme-demo
- Oxygen-Im-Phone-Ring - phone-incoming-call
- Oxygen-Sys-List-Match-No - completion-fail
- Oxygen-Sys-List-End - completion-rotation
- Oxygen-Sys-List-Match-Multiple - completion-success
- Oxygen-Sys-Error-Printing - print-error (not in the spec)
- Oxygen-Sys-File-Open-Foes - file-error (based on file-trash. Not in the spec. I interpreted "foes" as referring to an error.)

**Message sounds:**
- Oxygen-Im-Cant-Connect - message-connectivity-error (based on network-connectivity-error. Not in the spec.)
- Oxygen-Im-Error-On-Connection - message-connectivity-error-serious (same as Oxygen-Im-Cant-Connect but more specific. Not in the spec.)
- Oxygen-Im-Internal-Error - message-error (not in the spec)
- Oxygen-Im-Connection-Lost - message-connectivity-lost (based on network-connectivity-lost. Not in the spec.)
- Oxygen-Im-Network-Problems - message-conectivity-problem (not in the spec)
- Oxygen-Im-Contact-In - message-contact-in (not in the spec)
- Oxygen-Im-Contact-Out - message-contact-out (not in the spec)
- Oxygen-Im-Message-Out - message-sent-instant
- Oxygen-Im-Message-In - message-new-instant
- Oxygen-Im-Highlight-Msg - message-highlight (not in the spec)
- Oxygen-Im-Irc-Event - message-irc-event (not in the spec)
- Oxygen-Im-Low-Priority-Message - message-lowpriority (not in the spec. The name is composed of two words joined together because the "-" separates how specific the sound is, it's not meant as a simple way to separate words. In that case, if I added low-priority the code would always prefer this sound over the others.))
- Oxygen-Im-New-Mail - message-new-email
- Oxygen-Im-Sms - message-new-sms (not in the spec but it allows for more specific names)
- Oxygen-Im-Nudge - message-attention (not in the spec)
- Oxygen-Im-User-Auth - service-login

**Window management:**
- Oxygen-Window-All-Desktops-Not - window-unpin (not in the spec)
- Oxygen-Window-All-Desktops - window-pin (not in the spec)
- Oxygen-Window-Close - window-close
- Oxygen-Window-Maximize - window-maximized
- Oxygen-Window-Minimize - window-minimized
- Oxygen-Window-Shade-Down - window-shaded (not in the spec)
- Oxygen-Window-Shade-Up - window-unshaded (not in the spec)
- Oxygen-Window-Move-Stop - window-move-end
- Oxygen-Window-Move - window-move-start

**K3b:**
- Oxygen-K3B-Finish-Error - complete-media-error
- Oxygen-K3B-Finish-Success - complete-media-burn
- Oxygen-K3B-Insert-Medium - media-insert-request