This will allow us moving all the keybindings into gsettings and also
allows users to override keybindings when desired.

If a keybinding is for a "hardware" key, then we add a dummy first
entry. This allows users to define a further keybinding in g-c-c while
keeping the hardware key functional.

Note that we are not allowing multiple keybindings for custom
keybindings for now.

When migrating, any overriden keybinding is inserted into the first
slot of the default value. This means that for e.g. the audio keys we
are re-adding the hardware button. This could potentially create
collisions if the user changed the default binding *and* assigned the
hardware button to another purpose. Unfortunately, users will need to
correct this manually.