Steps to reproduce:
1. Open project named "_EditorWindowBug.zip"
2. Go to Examples->EditorWindowBug
3. Move "Title" window on Button (like shown in a GIF)
- Can't click on Button which is in "Title" window.Instead, it clicks on button behind him. (Also shown in a GIF).

Problem: Button within a GUI.Window inside an EditorWindow ignores mouse clicks.
Current result: First button in order is ignored.
Expected result: First button in order should be able to detect mouse clicks.

Reproduced on:5.4.0f3,5.3.5f1,5.2.3p3.