terminal/idev: add helper to match keyboard shortcuts

Matching keyboard shortcuts on internationalized keyboards is actually
non-trivial. Matching the actual key is easy, but the modifiers can be
used by both, the matching and the translation step. Therefore, XKB
exports "consumed-modifiers" that we use to figure out whether a modifier
was already used by the translation step.

The new IDEV_KBDMATCH() helper can be used to match on any keyboard
shortcut and it will do the right thing.