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

app-esp/src/hello_world/hello_world.c

@@ -1,7 +1,11 @@
 #include "hello_world.h"
 #include "lvgl.h"
+#include "ui/toolbar.h"
 
 static void app_show(TT_UNUSED App app, lv_obj_t* parent) {
+    lv_obj_t* toolbar = tt_toolbar_create_for_app(parent, app);
+    lv_obj_align(toolbar, LV_ALIGN_TOP_MID, 0, 0);
+
     lv_obj_t* label = lv_label_create(parent);
     lv_label_set_text(label, "Hello, world!");
     lv_obj_align(label, LV_ALIGN_CENTER, 0, 0);