LVGL improvements for TactilityC (#233)

- Expose various `lv_event`-related functions
- Expose `tt::lvgl::toolbar`-related functions
- Update sample app and improved docs

ExternalApps/HelloWorld/main/Source/main.c

@@ -6,7 +6,7 @@
  * Only C is supported for now (C++ symbols fail to link)
  */
 static void onShow(AppHandle app, void* data, lv_obj_t* parent) {
-    lv_obj_t* toolbar = tt_lvgl_toolbar_create(parent, app);
+    lv_obj_t* toolbar = tt_lvgl_toolbar_create_for_app(parent, app);
     lv_obj_align(toolbar, LV_ALIGN_TOP_MID, 0, 0);
 
     lv_obj_t* label = lv_label_create(parent);