Fix flatpak not using dark theme (#8964)

* Create set-dark-theme-variant.py

* Update entrypoint

* Update io.github.softfever.OrcaSlicer.yml

* Create uses-dark-theme.py

* Update MPFR to 4.2.2

---------

Co-authored-by: Noisyfox <timemanager.rick@gmail.com>
Co-authored-by: SoftFever <softfeverever@gmail.com>

flatpak/uses-dark-theme.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+
+import os
+import sys
+import gi
+gi.require_version("Gtk", "3.0")
+from gi.repository import Gtk as gtk
+
+
+if __name__ == '__main__':
+    forced = os.environ.get('ORCA_SLICER_DARK_THEME', 'false') == 'true'
+    settings = gtk.Settings.get_default()
+    prefer_dark = settings.get_property('gtk-application-prefer-dark-theme')
+
+    if not forced and not prefer_dark:
+        sys.exit(1)
+    else:
+        sys.exit(0)