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/llwindowmacosx.cpp

@@ -1756,13 +1756,6 @@ void LLWindowMacOSX::spawnWebBrowser(const std::string& escaped_url, bool async)
 	}
 }
 
-// Open a URL with the user's default web browser.
-// Must begin with protocol identifier.
-void LLWindowMacOSX::ShellEx(const std::string& command)
-{
-	std::system(("open " + command).data());
-}
-
 LLSD LLWindowMacOSX::getNativeKeyData()
 {
 	LLSD result = LLSD::emptyMap();