Compute tile geometry without relying on electric border mode

Quick tiling allows you tile a window so it covers one half or a quarter
of the screen. Electric border is basically interactive flavor of quick
tiling, i.e. it allows you to drag a window to the top screen edge to
maximize it.

Currently, it's confusing that tile geometry is computed based on the
electric border mode.

This change converts electricBorderMaximizeGeometry() in a helper that's
used to compute the tile geometry given the desired mode and output
containing the specified QPoint. With that, setQuickTileMode() won't
need to set electric border anymore, which makes tiling code more
comprehensible, but by not a lot unfortunately.


(cherry picked from commit 6e26494b)