feat: simple global control for backlight/brightness and volume (#6684)

radio/src/gui/gui_common.cpp

@@ -308,6 +308,11 @@ bool isSourceAvailable(int source)
             );
 }
 
+bool isSourceAvailableForBacklightOrVolume(int source)
+{
+  return checkSourceAvailable(source, SRC_SWITCH | SRC_POT | SRC_NONE);
+}
+
 bool isLogicalSwitchAvailable(int index)
 {
   LogicalSwitchData * lsw = lswAddress(index);