InkHUD: Allow non-system applets to subscribe to input events (#9514)

* Allow inkhud user applets to consume inputs with opt-in system
Adds a way for applets to subscribe to input events while keeping it off
by default to preserve compatibility and expected behaviours. Adds
example for use as well.
* Add check for nullptr on getActiveApplet uses
* Remove redundant includes
* Move subscribedInputs to protected
* More consistent naming scheme

src/graphics/niche/InkHUD/InkHUD.cpp

@@ -210,6 +210,12 @@ void InkHUD::InkHUD::prevApplet()
     windowManager->prevApplet();
 }
 
+// Returns the currently active applet
+InkHUD::Applet *InkHUD::InkHUD::getActiveApplet()
+{
+    return windowManager->getActiveApplet();
+}
+
 // Show the menu (on the the focused tile)
 // The applet previously displayed there will be restored once the menu closes
 void InkHUD::InkHUD::openMenu()