global: Remove special handling of the 'overlay-key' keybinding -
make it just a regular keybinding for the menu applet.
Unify keybinding schemas into org.cinnamon.keybindings.*
Rework cinnamon to handle new binding locations, as well as support handling
multiple bindings for all actions.

See related commits in:

https://github.com/linuxmint/cinnamon-desktop/commit/135fb6658e2c299412dac2e9fe4f253b3b27c12a
https://github.com/linuxmint/cinnamon-desktop/commit/78c84e9112a258f8304e4f731f074c2730c42856
https://github.com/linuxmint/muffin/commit/2c5ae187800c16c60156b5c0db119bc34ac40419
https://github.com/linuxmint/muffin/commit/66b6f369e2764e69b41a114bd84a40b4e03a729d
https://github.com/linuxmint/cinnamon-settings-daemon/commit/afd4a7d443d0a8dff01b0ba9bc9eba9e2f9944b4