Don't hardcode modifier masks in the wheel and keyboard input
controllers. Instead, hardcode accelerator strings and get the right
modifier mask from gtk_accelerator_parse() at runtime.

This turns e.g. "<Shift><Primary>" into Shift+Cmd on OS X and into
Shift+Ctrl on X11 and Windows.

Also rename the events accordingly and change the event names in the
default controllerrc.

Unrelated: reorder the actions so the ones with less modifiers are
listed first, and change the order of action matching to match the
actions with most modifiers (the last ones) first.