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/README.md

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+# /snap/local/launchers
+Here are the launchers, or wrapper programs to deal with some runtime-fixable problems for the snapped applications, like setting proper environmental variables in snap.
+
+In convention launchers are named _something_-launch, for dealing certain problem with _something_, and usually can be called in a stacked manner to consolidate their modifications.
+
+```yaml
+apps:
+  _app_name_:
+    command: foo-launch bar-launch _app_command_
+```
+

snap/local/launchers/gtk3-env-launch

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+#!/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 "${@}"