Improve the Keyboard tab and the KeyBindings editor

Summary:
- Change updateKeyBindingsList() to select the last edited keybindings
  scheme after closing the editor dialog
- Add a tooltip showing the path of each keybinding scheme, this should
  lessen the confusion when the scheme description and the file name of
  the .keytab file are different
- Add a button to the Keyboard tab to reset a scheme to its default
  values, this is only applicable for schemes that exist in both
  system-wide and user specific locations
- Move a lot of the construction of the keybinding editor widgets from
  EditProfileDialog to  KeyBindingEditor
- Change the wording from "key bindings list" to "key bindings scheme"
  to differentiate between a keyboard translator "scheme" and the key
  bindings "list" on the Keyboard tab

KeyBindingEditor:
- Make the KeyBindingEditor a QDialog and set Qt::WA_DeleteOnClose
- Disallow saving a scheme with an empty description
- Add a QLineEdit to show only the rows that have strings matchin...