Snap: fix theming (#3057)

Use gtk3 file chooser dialogs, mouse coursor theme if available and force fallback icon theme, fixes issue #2966

snap/local/launchers/gtk3-env-launch

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# This is the maintainence launcher for the snap, make necessary runtime environment changes to make the snap work here.  You may also insert security confinement/deprecation/obsoletion notice of the snap here.
+
+set \
+	-o errexit \
+	-o errtrace \
+	-o nounset \
+	-o pipefail
+
+# gtk-common-themes support
+export QT_QPA_PLATFORMTHEME=gtk3
+
+# Finally run the next part of the command chain
+exec "${@}"