This patch implements Drop operation on Wayland/Gtk+. That's because drop operations are part
of clipboard on Wayland and we use our own paste clipboard handler on Wayland (Bug 1282015).
Wayland drop data are provided by wl_data_device_listener, it provides us drag and drop callbacks
which we route to nsDragService module.
MozReview-Commit-ID: 9uGYPg9YF6P
When we perform copy -> paste in one Firefox process on Wayland we're locked because Wayland clipboard paste
operation just reads data from filedescriptor and does not run main event loop.
A solution is to use Gtk+ shortcut here, when clipboard selection owner is the same as data receiver.
Gtk+ then does not go through X11/Wayland but calls clipboard data getter callback directly,
which we can use on Wayland because it also does not main event loop and the operation
stays synchronous.
MozReview-Commit-ID: G8myCBUSzxb
