Refactor && fix hotkeys definitions

[let4be]

It's a big mess and it's outdated... some of hotkeys don't work

• [x] refactor and fix bugs
• [x] integrate this with new modals, because we don't want shortcuts to trigger when a modal is opened
• [x] add support for multi-level hotkeys, e.g. press g and then press i to go inbox if second key wasn't pressed in X msec, unbind it
• [x] display navigation hints for multi-level hotkeys
• [x] properly display legend

after this, integrate: https://github.com/lavab/web/issues/545

[let4be]

now user can just press g(goto) and he sees this:

this thing timesout after 3 sec, giving user a short hint what to press

@andreis @flixi @vLooz