desktop: prompt to power off on back button long press (#1279)

* desktop: prompt to power off on back button long press
   Implemented by launching Power Settings app, passing "off" as arg to
   directly jump to "power off" scene.

applications/desktop/views/desktop_events.h

@@ -8,6 +8,7 @@ typedef enum {
     DesktopMainEventOpenMenu,
     DesktopMainEventOpenDebug,
     DesktopMainEventOpenPassport, /**< Broken, don't use it */
+    DesktopMainEventOpenPowerOff,
 
     DesktopLockedEventUnlocked,
     DesktopLockedEventUpdate,

applications/desktop/views/desktop_view_main.c

@@ -53,6 +53,8 @@ bool desktop_main_input(InputEvent* event, void* context) {
             main_view->callback(DesktopMainEventOpenDebug, main_view->context);
         } else if(event->key == InputKeyLeft) {
             main_view->callback(DesktopMainEventOpenFavoriteSecondary, main_view->context);
+        } else if(event->key == InputKeyBack) {
+            main_view->callback(DesktopMainEventOpenPowerOff, main_view->context);
         }
     }