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/base-backend.cpp

@@ -33,7 +33,7 @@
 
 #include "gui/message.h"
 
-void BaseBackend::displayMessageOnOSD(const char *msg) {
+void BaseBackend::displayMessageOnOSD(const Common::U32String &msg) {
 	// Display the message for 1.5 seconds
 	GUI::TimedMessageDialog dialog(msg, 1500);
 	dialog.runModal();