Implemented Files app (#33)

- Created Files app to browse PC and ESP32 files.
- Refactored toolbars so it's now a proper widget and allows for changing its properties from the app
- Toolbar now has extra action buttons
- Settings app now has a proper icon
- Minor cleanup in Desktop app

README.md

@@ -28,6 +28,11 @@ Creating a touch-capable UI is [easy](https://docs.lvgl.io/8.3/get-started/quick
 
 ```c
 static void app_show(TT_UNUSED App app, lv_obj_t* parent) {
+    // Default toolbar with app name and close button
+    lv_obj_t* toolbar = tt_toolbar_create_for_app(parent, app);
+    lv_obj_align(toolbar, LV_ALIGN_TOP_MID, 0, 0);
+    
+    // Label widget
     lv_obj_t* label = lv_label_create(parent);
     lv_label_set_text(label, "Hello, world!");
     lv_obj_align(label, LV_ALIGN_CENTER, 0, 0);