Make context drawer interactive on mobile (#478121)

Reason for the change

Fix https://bugs.kde.org/show_bug.cgi?id=478121 (Elisa mobile UI: playlist drawer cannot be closed anymore)

Test plan

1. Install Elisa on Plasma Mobile. (I use the Manjaro ARM elisa-qtmm package, but I doubt it depends on the multimedia backend.)
2. Open Elisa.
3. Touch (click) the "Show playlist" button.
4. Try to close the playlist drawer by dragging it to the side with the finger.

Bugs fixed

BUG: 478121