Add Ctrl-X cut and paste-above-selection

Cut (Ctrl-X) marks the selection on the clipboard with a `cut` flag; a
paste of a cut from a real playlist is a *move* (single-undo across
playlists via move_tracks_between_playlists; same-playlist reorders),
while a copy — or a cut from the library, which has nothing to remove —
just adds. Paste now inserts *above the selected track* rather than
appending. Cut/Paste added to the track context menu; parse_tracks_mime
now tolerates a None/empty clipboard.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -69,7 +69,7 @@ if it's missing, you'll be asked to pick a font on first run.
 ## Keys
 
 Space play/pause · ←/→ previous/next · Ctrl+B column browser ·
-Ctrl+I get info · Ctrl+, preferences · Ctrl+C/Ctrl+V copy/paste tracks ·
+Ctrl+I get info · Ctrl+, preferences · Ctrl+C/Ctrl+X/Ctrl+V copy/cut/paste tracks ·
 double-click sidebar art for a big art window
 
 ## App icon