New button to populate destination position, other assorted code upgrades

[GrandyB]

Added:

• GPL2 license header.
• Comments!
• A button which populates the destination X/Y with the source's current position.

CHANGED:

• Renamed and split set_vis into set_visibility_bool(key, bool) and set_visibility(key, val, cmp), for the two use cases.
• Localisation text.

NOTES (may add as issues/comments):

• If adding relative (rather than absolute) motion at some point, the new button won't be very useful
• Would be good to safely store the initial width/height of the source somewhere, so that if you give a custom starting size and then un-tick the option, it should revert back to its original size

[GrandyB]

NOTE: There's an extra item in the en-US.ini localisation file: DestinationGrabPosition="Populate destination with current source position" Will need this item translated/added to the other locale as well.

[CatxFish]

I'll handle the Mandarin part.