Bug 626499 - GtkClipboard unnotified on change of OS X pasteboard owner

Bug 626499 - GtkClipboard unnotified on change of OS X pasteboard owner

pasteboardChangedOwner is not called as reliably as we'd want to get it,
so keep track of [pasteboard changeCount] and drop clipboard ownership
when a change happened. Also better unset the clipboard content redundantly
in a few places rather than missing one, and reorder the code in
gtk_clipboard_set_contents() so that the new aggressive unsetting
won't unset the clipboard under our feet when we call
[pasteboard declareTypes].
(cherry picked from commit f2b74db5dcbd28a1e27431f413c66d2a5d50b2bd)