Bug fix -- Right Click Menu Crash in Edit Tag Window

[Eoin-ONeill-Yokai]

I fixed a crash that complained about adding a list and a map together. The following is the crash report I received:

TypeError: unsupported operand type(s) for +: 'map' and 'list'
------
Traceback (most recent call last):

  File "/usr/lib/python3/dist-packages/quodlibet/qltk/edittags.py", line 632, in __popup_menu
    [1])

TypeError: unsupported operand type(s) for +: 'map' and 'list'

Converting the map to a list allows the addition of these elements, but I'd like to hear if this is actually the desired behavior? Never-the-less, this PR solves the bug and seems to run well on my local repository.

This is my first commit to this project, so hopefully it meets your standards.

[lazka]

Sentry issue: QUODLIBET-9C