fix GUI depth

[Hirashi3630]

fixing this would also fix #28

[Hirashi3630]

Solution idea: add some kind of manager that gives corresponding depth values to each window when drawn

[Hirashi3630]

this problem is deeper than expected... via:

https://forum.unity.com/threads/corrected-gui-button-code-works-properly-with-layered-controls.96563/#post-629284 http://answers.unity.com/answers/1628530/view.html

[Hirashi3630]

TODO:

• rewrite all remaining GUI controls such as TextField, Slider... same way as I did with button in 51b80ea (or find a way to block everything with the background window's box)
• remove ability to drag windows through other windows