main: allow users to override QT_QPA_PLATFORMTHEME

commit 971baf2e added
code to make Qt applications look like GNOME ones.

That makes it impossible for users to use custom
platform themes though.

This commit addresses the problem by only setting
QT_QPA_PLATFORMTHEME if it isn't already set.

https://bugzilla.gnome.org/show_bug.cgi?id=762681