Custom key bindings should support modifier keys

[kevinfoley]

Several default keyboard bindings in TFE use modifier keys. For example, by default, system menu is Alt-F1.

The Settings interface does not allow us to use a key combination that includes a modifier key. The first key we press is immediately set as the binding; for example, if we try to bind Alt-F1, the binding will either get set to Alt or to F1 (whichever we pressed first).