Qt: Add Gzipped

WX supported this, I don't see why not add to Qt. Though no idea why anyone wants to use it above CSO or CHD if you look at filesize.

pcsx2-qt/MainWindow.cpp

@@ -46,11 +46,12 @@
 #include "svnrev.h"
 
 static constexpr char DISC_IMAGE_FILTER[] =
-	QT_TRANSLATE_NOOP("MainWindow", "All File Types (*.bin *.iso *.cue *.chd *.cso *.elf *.irx *.m3u *.gs *.gs.xz);;"
+	QT_TRANSLATE_NOOP("MainWindow", "All File Types (*.bin *.iso *.cue *.chd *.cso *.gz *.elf *.irx *.m3u *.gs *.gs.xz);;"
 									"Single-Track Raw Images (*.bin *.iso);;"
 									"Cue Sheets (*.cue);;"
 									"MAME CHD Images (*.chd);;"
 									"CSO Images (*.cso);;"
+									"GZ Images (*.gz);;"
 									"ELF Executables (*.elf);;"
 									"IRX Executables (*.irx);;"
 									"Playlists (*.m3u);;"