Fix Tails Cloner bug: button sensitivity or labels don't update if source or targets change #20042
Closes #20042 (closed).
The first added line to def on_target_changed() fixes the labels on GUI elements to reflect whether a new target is a tails or non-Tails USB. The second added line fixes the sensitivity of the OK button so it updates for the new target.
Cherry picked from !1292 (closed) to isolate the bug-fix from the feature additions.