wacom: Implement support for stylus button custom keyboard shortcut

Requires gsettings-desktop-schemas!76 for gsettings

Pending mutter!3649

This implements support for the stylus custom keyboard shortcuts implemented in the above merge requests. A new 'Custom Keybinding' row was added that will display the currently saved shortcut and includes a button to bring up the editor window that will handle changes to the shortcut.

The 'keybinding_edited' method introduced in cc-wacom-stylus-action-dialog makes use of code from keyboard-shortcuts's normalize_keyval_and_mask method. I had initially tried sharing the method to make it a lot cleaner but I genuinely could not get it to compile. I'm not sure if I missed something that would've made it work but if there is a way to properly share the method it would be great and save on a lot of clutter. Solved Thank you Peter

cc @carlosg, @whot