[Util] ShortcutManager::SetShortcut() isn't const anymore.
[Util] Support global shortcuts in shortcut manager.

Rels #1278.
[Azoth] Settable global shortcut for unread msgs for #1278.
[Util] Documented new ShortcutManager's members.

Those for the global shortcuts.
Merge branch 'master' of github.com:0xd34df00d/leechcraft