Add mechanism unlinking functionality to buildingplan plugin

[Bumber64]

Adds ability to unlink and free mechanisms from buildings.

Replaces DFHack/scripts#1016

[myk002]

This is what I see, which is a bit more unstable than I'd like:

edit: replaced video with version that shows the current screen dimensions

simplescreenrecorder-2024-03-21_23.49.13.mp4

[Bumber64]

This is what I see, which is a bit more unstable than I'd like:

Should be fixed.

Also fixed "Free all" button disappearing when freeable mechanisms are scrolled offscreen.

[Bumber64]

Looks like all OverlayWidgets use default_pos instead of frame t/b/l/r even if they shouldn't be reposition-able? This generates a pos value in overlay.json. If we have to change the default in the future (e.g., because UI changes,) it's going to keep using the old value in every user's json.

We probably shouldn't save/load positions for OverlayWidgets with overlay_only = true. That would mean we wouldn't respond to overlay position <overlay> either? That would stop the user from manually fixing things, though.

version can be used, per Discord.

[myk002]

This looks great! Thanks!