It turns out XKB has keysyms for these for no real reason, and I
was hitting those instead of the <Primary><Alt>F1 path. This is
ridiculous, but key, so is the entirety of XKB.
This took an embarassingly long time to figure out and debug.
Once mutter is started from weston-launch on its own VT, there is
no way to change VT again (for example to actually start an application),
because the keyboard is put in raw mode.
So introduce some keybindings mimicking the standard X ones (Ctrl+Alt+Fn)
that switch the VT manually when activated.
https://bugzilla.gnome.org/show_bug.cgi?id=705861
