GUI: Improve handling of button presses while moving the cursor

Keep track of where a mouse press started when deciding how mouse-over
and mouse-up should behave.

This handles the following situations:

1) If a mouse press starts outside a button (e.g. the UI background),
mouse-up inside the button has no effect. Previously, it triggered a
button click.
2) If a mouse press starts inside a button, the cursor moves outside of
its region, then back inside, the button will show as pressed.
Previously, it showed as highlighted instead of pressed.

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