Implement reversible span selection behavior

[hitex]

Hello, @domferr

First of all, thank you for amazing extension!

My PR implements a feature that I am missing from FancyZones - reversible span selections. Currently when spanning the window, if you move too much with the mouse - you need to cancel spanning and start over, which is really annoying. In this PR you can change the spanning area freely.

Heres how the logic is implemented:

• when spanning starts, the starting tile is stored as an anchor
• as the pointer moves, selected rectangle is recomputed from anchor + current tile
• tile previews are rebuilt from original tile geometry each time the new tile is hovered
• anchor/hover state is cleared when span mode ends

Here is the video recording of the behavior:

reversible-span.mp4

It works on Gnome 49. Did not test older versions yet because I want to get your feedback first and see if you are willing to incorporate this change. Let me know what you think.

PS. I really missed the tests, did you consider adding them at some point?