Changes:
* Add KeyboardShortcutsCommand and window
* Add SelectAccelerator window
* Replace modules/gui.cpp functions with app::KeyboardShortcuts and
app::Key with the logic to load/save/handle keyboard shortcuts
* Change ui::Accelerator concept: now it represent just one keyboard
shortcut, not a set of shortcuts
* Remove ui::Accelerator from ui::MenuItem, now the key is associated
in app::AppMenuItem and it's a app::Key
* Add Command::onGetFriendlyName() to get a user friendly name of the
command depending on its parameters
