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/llwindow/llwindow.h

@@ -168,7 +168,7 @@ public:
 	virtual void updateLanguageTextInputArea() {}
 	virtual void interruptLanguageTextInput() {}
 	virtual void spawnWebBrowser(const std::string& escaped_url, bool async) {};
-	virtual void ShellEx(const std::string& command) {};
+	static int ShellEx(const std::string& command);
 
 	static std::vector<std::string> getDynamicFallbackFontList();