Further/extensive testing by someone who actually uses high-DPI monitor has indicated that enabling AA_EnableHighDpiScaling (via the QT_AUTO_SCREEN_SCALE_FACTOR environment variable) works and "is a must" on GNOME as well as a number of other desktop environments (including Budgie, Cinnamon, Deepin, KDE, and XFCE). The only exception to this, reportedly, was MATE -- which, I've been told, works/looks better with auto-scaling disabled...

Accordingly, this PR enables on auto-scaling for all platforms/environments except for MATE and Qubes-OS (which, as mentioned in the in-code comment, continues to apply inconsistently).

Note that Qt's auto-scaling can still be enabled or disabled by setting the QT_AUTO_SCREEN_SCALE_FACTOR environment variable to 1 or 0 (respectively), allowing users to override Qt's behavior independently of this change and/or on a per-application basis as desired.