frames: Always start grab operation on left click

When left-clicking the frame border with the titlebar being
off-screen, rather than starting the expected grab operation the
window menu was popped up.
This behavior is pretty confusing, especially since the menu button
was removed from the default layout, making right-clicking the only
way to get to the window menu.

https://bugzilla.gnome.org/show_bug.cgi?id=652369