Added a Dropbox version control plugin for Dolphin.

This Dropbox plugin is based on the work of:
Sergei Stolyarovs - https://bitbucket.org/cancel/dolphin-dropbox-plugin
Thomas Richards - http://trichard-kde.blogspot.co.at/2010/12/introducing-dropbox-integration-for.html

What I have done:
- Ported the old source code to the newer KVersionControlPlugin2 interface
- Use the Dropbox client to form the context menu more dynamically
  (If the Dropbox guys add a new feature to their client, Dolphin can make use of it automatically)
- Fixed a crash (Dolphin-4.8.2 segfaults when a file with special characters is present)
- Replaced the item version changed timer with a file system watcher -> No useless updates and immediate update on changes
- A lot of code/coding style related changes
- Made symlinks work with Dropbox

BUG: 298199
BUG: 264717
FIXED-IN: 4.13.0
REVIEW: 114812