GUI: view enter, exit callbacks. (#326)

applications/gui/view.h

@@ -42,6 +42,12 @@ typedef bool (*ViewInputCallback)(InputEvent* event, void* context);
  */
 typedef uint32_t (*ViewNavigationCallback)(void* context);
 
+/* View callback
+ * @param context, pointer to context
+ * @warning called from GUI thread
+ */
+typedef void (*ViewCallback)(void* context);
+
 /* View model types */
 typedef enum {
     /* Model is not allocated */
@@ -92,6 +98,18 @@ void view_set_previous_callback(View* view, ViewNavigationCallback callback);
  */
 void view_set_next_callback(View* view, ViewNavigationCallback callback);
 
+/* Set Enter callback
+ * @param view, pointer to View
+ * @param callback, callback
+ */
+void view_set_enter_callback(View* view, ViewCallback callback);
+
+/* Set Exit callback
+ * @param view, pointer to View
+ * @param callback, callback
+ */
+void view_set_exit_callback(View* view, ViewCallback callback);
+
 /* Set View Draw callback
  * @param view, pointer to View
  * @param context, context for callbacks