Fix up ShellEx

It's now crossplatform wrapper for std::system opening of a file/path/url.
It returns an int just like std::system, 0 means it worked.
It's static so you don't need to gViewerWindow->getWindow() anymore,
honestly it should've been static in the first place.

Clean up other code to use this function because duplicate code sucks.

# Conflicts:
#	indra/llwindow/llwindowmacosx.h
#	indra/llwindow/llwindowsdl2.cpp

indra/newview/lgghunspell_wrapper.cpp

@@ -948,7 +948,7 @@ void lggHunSpell_Wrapper::editCustomButton()
 			//glggHunSpell->addWordToCustomDictionary("temp");
 	}
 	
-	gViewerWindow->getWindow()->ShellEx(dicdicpath);
+	LLWindow::ShellEx(dicdicpath);
 }
 
 void lggHunSpell_Wrapper::setSpellCheckHighlight(BOOL highlight)