window: "Hide" edge resistance behind modifier key

Quoting the main commit:

Aligning windows manually with other windows has become less important
since the advent of tiling. This decreases the usefulness of edge
resistance, which in fact many users perceive as lag nowadays.

Account for that by limiting resistance to screen and monitor edges by
default, and only include windows when the control key is pressed.

https://bugzilla.gnome.org/show_bug.cgi?id=679609

My first approach was to disable all edge-resistance by default as in !455 (closed), but while I don't miss edge resistance on windows at all, the lack of any resistance on screen and monitor edges felt extremely weird.

Fixes #181 (closed)