Added PBubbleWindow, a backport of GtkBubbleWindow from GTK+ 3.9
Renamed GtkBubbleWindow to PBubbleWindow
Changes needed to compile PBubbleWindow
Ensure that the methods of PBubbleWindow are public and can be introspected.
Private class. Introspection information.
Merge pull request #1 from felipeerias/bubblewindow

Backport `GtkBubbleWindow` from 3.9