Standalone app api v3 (#2772)

Added file io, and updated some ui elements.
Also added Digital Rain standalone app for an example.

firmware/application/theme.cpp

@@ -4,6 +4,12 @@ namespace ui {
 
 ThemeTemplate* Theme::current = nullptr;
 
+void Theme::destroy() {
+    if (current != nullptr)
+        delete current;
+    current = nullptr;
+}
+
 ThemeTemplate* Theme::getInstance() {
     if (current == nullptr) SetTheme(DefaultGrey);
     return Theme::current;