scripts/qsscheck: Also check default stylesheets

[Holzhaus]

Add support for qss stylesheets added in PR #2362.

[uklotzde]

@ronso0 Any comments on this?

[ronso0]

nope. Can't evaluate the script, and I suppose @Holzhaus tested it sufficiently.

[Holzhaus]

@uklotzde We already found some issues using that script, e.g. #2387, #2374 and the qss syntax errors fixed by #2342. For the current master branch, there are no false positives. The script is certainly not perfect, but it can help keeping the qss scripts clean.

If the script complains and we determine that it's a false positive, we can still ignore the warning and merge anyway, and also fix the script itself.