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

gui/updates-dialog.cpp

@@ -113,7 +113,7 @@ UpdatesDialog::UpdatesDialog() : Dialog(30, 20, 260, 124) {
 	int buttonPos = _w - buttonWidth - 10;
 
 	_proceedButton = new ButtonWidget(this, buttonPos, _h - buttonHeight - 8, buttonWidth, buttonHeight,
-				_("Proceed"), nullptr, kProceedCmd, Common::ASCII_RETURN);
+				_("Proceed"), Common::U32String(""), kProceedCmd, Common::ASCII_RETURN);
 }
 
 void UpdatesDialog::handleCommand(CommandSender *sender, uint32 cmd, uint32 data) {