Keeping track of mapped keyboard hotkeys

Closes #278

`keyCapture.initCaptures` now returns all successful keybindings. It also stores them itself in an `activeKeys` property.

The navbar buttons make use of this and display their hotkey in their title now. I think this wouldn't play too nice right now when keybindings change on the fly, but this won't happen now anyway. Maybe optimize later, if there's a need to.

Minor fix for the titles: They sit now on the element (in this case a button) and not on the icon. This makes it easier to trigger the title - before you really had to hover the icon which felt a bit awkward.

`keyCapture.activeKeys` is also used to produce a little table of all active keybindings, which are displayed in the new `help` section. There's nothing inside of it apart from the hotkey list as of yet.