added a settingspage
Initial commit. Feature is basically working. No UI, pretty hacky.
Some tidying up
Added signal to update quick accessors in the main window
Removing some cruft
Quick Accessors can be set up by right clicking on buffers in the bufferlist
Tidied up some signalling
Internal changes to facilitate using a SettingsPage to change Quick Accessors.
Make QuickAccessorsPage derive from ShortcutsSettingsPage and mostly work.
Deleted references to old-style quick accessors
Delete some debug messages
Fixed up some indentation
Remove overridden functions that don't change anything
If KDE support is enabled, add a separate menu entry to edit quick accessors
Fixed messed up formatting
Resolve conflicts between normal shortcuts and QuickAccessors.

Now both shortcuts pages see all ActionCollections but filter out the irrelevant ones.
(Should we remove the arguments from the constructor?) And resolve conflicts using the
existing mechanisms. A signal is now emitted when an ActionCollection's action is changed,
and the ShortcutsModel listens for this to update shortcuts changed from another page.
We can't customise the KDE page, so shortcuts overwritten there are currently not even
warned about.
Fixed up some formatting errors
Fixed a crash caused by deleting the wrong thing in
setupQuickAccessors()
call setupQuickAccessors() before showing the shortcuts dialog
Fixes Buffer Shortcut MenuItem not showing in inactive buffers