windowMenu: Do a better job with faking the source actor

windowMenu: Do a better job with faking the source actor

The 0x0 dummyCursor works well when the menu pops up directly underneath
the pointer (e.g. when triggered by right-clicking the titlebar) or by
keyboard, but not when triggered by the menu button - the menu does not
point to the center of the button's bottom edge, and unless the user
keeps holding the mouse button while moving into the menu, the menu will
be dismissed immediately on button release.
Address these issues by using the button geometry to overlay the window
button with an appropriately sized actor that acts as a proper sourceActor,
to make the window menu behavior consistent with other shell menus.

https://bugzilla.gnome.org/show_bug.cgi?id=731058