Implement resizing a layout with the mouse

[werererer]

We have to have this feature because i3 and bspwm does it and we don't want to fall behind on that aspect ;). The problem is though that I personally don't use it since I am a pure keyboard user. So any help would be appreciated.

If you want to contribute just write me here or on discord/reddit.

[werererer]

This will be the main focus for 0.4.2. A lot of things need to be changed before this will eventually work.
This depends on the follow
#30 new border logic (done)
and also the arrange algorithms must become more modular which will also remove most of the flickering.