InkHUD: add offline map tile backgrounds and zoom controls

src/graphics/niche/InkHUD/Applet.h

@@ -128,6 +128,8 @@ class Applet : public GFX
 
     virtual bool approveNotification(Notification &n); // Allow an applet to veto a notification
 
+    virtual class MapApplet *asMapApplet() { return nullptr; } // Returns non-null only for MapApplet and its subclasses
+
     static uint16_t getHeaderHeight(); // How tall the "standard" applet header is
 
     static AppletFont fontSmall, fontMedium, fontLarge; // The general purpose fonts, used by all applets