gtk: Stop using modifier intents

gtk: Stop using modifier intents

Reviewing the existing settings, the only backend with
some differences in the modifier intent settings is OS X,
and we would rather have that implemented by interpreting
the existing modifiers in the appropriate way.

                X11      Wayland  Win32    OS X

primary         ctrl     ctrl     ctrl     mod2
mnemonic        alt      alt      alt      alt
context menu    -        -        -        ctrl
extend sel      shift    shift    shift    shift
modify sel      ctrl     ctrl     ctrl     mod2
no text         alt|ctrl alt|ctrl alt|ctrl mod2|ctrl
shift group     varies   -        -        alt

GTK now uses the following modifiers:

primary         ctrl
mnemonic        alt
extend sel      shift
modify sel      ctrl
no text         alt|ctrl

The context menu and shift group intents were not used
in GTK at all.

Update tests to no longer expect <Primary> to roundtrip
through the accelerator parsing and formatting code.