Remember mouse position when changing outputs

[rhubinak]

When switching between outputs, the cursor is centered on the new output. This can be annoying when, for example, copypasting stuff between two windows (typically, browser and command line) and leads to more mouse movement. Therefore, please add a new mouse_warping setting: remembered, which will remember the last mouse position on output.

This tracks the i3 issue i3/i3#4981 and the linked PR i3/i3#5083

Lemme know if this makes sense and if so, I'll try to remember my uni C courses and piece together a PR.