Fixes #234

This fixes some weirdness on some platforms, where Qt.openUrlExternally
does weird stuff.

While it does improve performance a bit, it is simply bad for most
distributions, since they don't know and probably don't want to
recompile Nheko. So default it to off. Anyone who wants it can still
toggle it on and it is almost no maintenance cost.

Correct a typo in SnackBar.cpp

Thank you Sorunome for reporting.

Fix not being able to change theme