Extract GtkWindow dragging and titlebar actions

• Remove GtkWindow assumption from gtk_widget_consumes_motion()
• Expose gtk_window_update_toplevel() in gtkwindowprivate.h (carry over the #2689 (closed), not sure why that call is needed in the first place)
• Add a widget that can be dragged, handles double click, right click, middle click and has the fallback window menu.
  • Fix fallback menu positioning
  • Remove "Always on top" item as the API it used is gone
• Make GtkHeaderBar contain the new widget and a GtkCenterBox instead of having GtkCenterLayout
• Remove the dragging and titlebar actions from GtkWindow. The click gesture remains as it's also used for resizing, the two drag gestures are completely gone.

#2242 (closed)