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.h

@@ -0,0 +1,22 @@
+
+#ifndef SPECTRE_PLATFORM_UNIX_XLIB_H
+#define SPECTRE_PLATFORM_UNIX_XLIB_H
+
+#include <string>
+#include <X11/Xlib.h>
+
+// Wrapper for some Xlib functions.
+
+namespace sp { namespace Xlib {
+
+void init();
+
+void shutdown();
+
+::Display* getDisplay();
+
+::Atom getAtom(const std::string& name, bool onlyIfExists = false);
+
+} } // namespace sp::xlib
+
+#endif /* SPECTRE_PLATFORM_UNIX_XLIB_H */