This means we can get rid of more cases in the ugly switch in navigator.
Guard against non mutter action names, but apply the override even when we can't
get the id.
For some reason restore-shortcuts (`<Super>Escape`) returns
KeybindingsAction.NONE.
Also remove setKeybinding
This is the common functions needed for both the extension side and prefence UI
side, so it goes in `settings.js`.
`Settings.findConflicts` searches for keycombo conflicts between our own
bindings and the built in Gnome Shell and mutter bindings, returning an array
like this:
```
[{ name: our own action name,
conflicts: array of conflicting names
settings: the settings schema the conflicting are part of
combo: the key combo as generated by keystrToKeycombo},
...]
```
keystrToKeycombo now handles `Above_Tab` properly.
Try to make it a bit more "action oriented":
An action exists independently of its binding. It could conceivable be invoked
by other means too.
Some weirdness from the marriage remain to weed out, but this is a working
stepping stone.
