Add level debugger screen

The level debugger is toggleable in playtesting mode by pressing Y.
You can toggle whether or not the game is paused inside of the debugger
by pressing TAB. The debugger screen allows you to see entity and block
properties, and allows you to move them around.

desktop_version/src/LevelDebugger.h

@@ -0,0 +1,14 @@
+#ifndef LEVELDEBUGGER_H
+#define LEVELDEBUGGER_H
+
+#include <SDL.h>
+
+namespace level_debugger
+{
+    bool is_pausing(void);
+    bool is_active(void);
+    void input(void);
+    void render(void);
+}
+
+#endif /* LEVELDEBUGGER_H */