gui z order fixed; gui menu item's content area fixed; menu item's clicked is only called once
Merge remote-tracking branch 'remotes/origin/master' into Resource_Manager
- fixed some TODO: close file somehow
fixed tabs.
Merge pull request #129 from nem0/fs_refactor

- fixed some TODO: close file somehow
partly implemented material manager
Merge remote-tracking branch 'remotes/origin/master' into Resource_Manager

Conflicts:
	src/core/tcp_file_device.cpp
	src/graphics/material.cpp
	src/graphics/material.h
partly implemented Shader Manager
partly implemented Texture Manager