add multimedia key and dock icon click support

These 2 commits add
 - multimedia keys handling: ability to inhibit those keys emission (block iTunes launching) and to process them in gtk, via a new signal on the GtkOSXApplication : 'MultiMediaKey'.
- clicks on the dock or relaunch by the user (```NSApplicationDelegate``` protocol function ```applicationShouldHandleReopen:hasVisibleWindows:```)

Regarding multimedia keys handling, the events are also seen in the ```global_event_filter_func``` but I would like to give the application the ability to swallow the event, which can only be done via an event tap, (and then if it's swallowed it won't appear in the ```global_event_filter_func```).

The 'Reopen' event is necessary to handle showing again a gtk window hidden by clicking the close button, when the user clicks on the dock. This is what non document based applications like iTunes and the activity monitor do.