windowMenu: Only disable "Always on Top" for fully maximized

Since (mutter) commit 49400657caae27 we disable the "Always on Top"
action for maximized windows, however when commit e7af257814b1
moved the window menu into the shell, this behavior was accidentally
extended to partially maximized (including tiled) windows.
As it can be desirable in this case to keep the window visible while
interacting with a different one, restore the previous behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=737382