GUI: U32: Convert majority of code to use U32

This commit also includes some additional major changes.

- Return U32String from TransMan getTranslation()
- Change tooltips to be U32Strings
- Remove temporary code of convertToU32String
- U32 Support various dialogs (Browser, FileBrowser, Messages, Chooser, etc)
- U32 for music plugins
- U32 for OSD messages
- Add some helper functions to ustr.cpp
- Change default tooltips from nullptr & 0 -> ""
- Some runtime exceptions may occur when changing languages due to incorrect String::Format

backends/graphics/sdl/sdl-graphics.cpp

@@ -419,7 +419,7 @@ Common::Keymap *SdlGraphicsManager::getKeymap() {
 #ifdef USE_SCALERS
 	struct ActionEntry {
 		const char *id;
-		const char *description;
+		U32String description;
 	};
 	static const ActionEntry filters[] = {
 			{ "FLT1", _s("Switch to nearest neighbour scaling") },