gtkwindow: Only allow unrestricted positioning to text handle popovers

gtkwindow: Only allow unrestricted positioning to text handle popovers

This behavior has been made optional on add_popover() time, text handles
will keep being able to overflow the window, in order to allow text
selection on views too close to the window edge.

Regular GtkPopovers are reinstaurated to the previous size positioning
logic though, that is, limited by the visible area of the window.