Add 'glue' mode to S&T to keep window's original size

[humdinger]

Not sure if this would be very useful for stacked windows, but it would be for tiled windows:

Currently, if you tile a window to another, the windows always get resized to match the bigger of the two windows:

Sometimes it may be nicer to keep the size of the smaller window, though, and just stick it to the border of the bigger window. Here, the small tools window of ArtPaint.
In that case it wouldn't be "tiling" really, but "glueing":

We could use SHIFT additionally to the OPT key to initiate that special glue-mode.

[humdinger]

Tiling

[humdinger]

Glueing