Operators ignored edges along the screen-layout bounds. They should've ignored
those along window bounds instead.
Although the global areas can not be joined/split, the adjacent areas can. So
the menu should still be shown.
Had to change the return value of area joining operator, so that an error report
can show up when trying to join over a global area edge. Think this is fine to
do, but you never know with such stuff.
Preferably we'd gray out the "Join Area" item in the menu when clicking on the
edge of a global area. Unfotunately the operator uses coordinates passed as
operator properties to find the right edge/areas, which we cannot access from
the poll callback.
bea364fe2a