Platform/Unix: Rename X11SharedDisplay to Xlib, and remove Display member variable from all classes.

We now initialize/destroy the display in Xlib::init/shutdown that is called in UnixApplication::init/shutdown and
therefore is valid through the whole lifetime. So no need for classes to keep references.

source/Platform/Unix/Xlib.cpp

@@ -0,0 +1,34 @@
+
+#include <assert.h>
+#include "Xlib.h"
+
+namespace sp { namespace Xlib {
+
+namespace _priv {
+	// Define a global display (for simplicity, we always connect to local one.)
+	::Display* display = NULL;
+};
+
+void init()
+{
+	_priv::display = XOpenDisplay(NULL);
+}
+
+void shutdown()
+{
+	XCloseDisplay(_priv::display);
+	_priv::display = NULL;
+}
+
+::Display* getDisplay()
+{
+	assert(_priv::display != NULL);
+	return _priv::display;
+}
+
+::Atom getAtom(const std::string& name, bool onlyIfExists)
+{
+	return XInternAtom(getDisplay(), name.c_str(), onlyIfExists);
+}
+
+} } // namespace sp::Xlib