feat: HDMI refresh boost per display, with a Saved displays screen

Rates are saved per display (EDID ID) and loaded by the hotplug script
for whichever display is connected. The item is on only while the
connected display is boosted; unticking removes just that one. The app
lists the saved displays and removes any of them (hdmi-forget).

ui/steamify-ui

@@ -289,6 +289,11 @@ class Backend(QObject):
         """The display's own EDID and the old rate again (hdmi-reset event)."""
         self._start(["hdmi-reset", conn, str(w), str(h), "%g" % hz], password)
 
+    @Slot(str, str)
+    def hdmiForget(self, password, display):
+        """Removes a saved display (hdmi-forgotten event)."""
+        self._start(["hdmi-forget", display], password)
+
     def _read(self, p):
         self._buf += bytes(p.readAllStandardOutput())
         while b"\n" in self._buf: