multimedia keys handling

new signal 'MultiMediaKey' emitted when a multimedia
(play-pause, next, previous, volume up/down) key is pressed

They are captured via a global event tap to swallow requested
multimedia keys (so that default applications don't get them).

The eventtap runs in a separate thread to not disturb the native
menus: they get laggy and can even freeze the screen if the
eventtap is added to the main run loop.
new application signal 'Reopen' to forward clicks on the dock to the gtk application

it forwards the NSApplicationDelegate applicationShouldHandleReopen:hasVisibleWindows:
callback. Will return YES by default, as expected.