[gui] added colordiffuse attribute on texture definition

This PR adds color diffuse attribute for texture definitions.

It's based on the PR #268 and gets rid of the borderdiffuse attribute. also it don't override m_diffuseColor as @jmarshallnz has suggested.

Example:
```
<texture colordiffuse="FFFFAAFF">texture.png</texture>
<texture colordiffuse="$VAR[colordiffuse]">texture.png</texture>
<texture colordiffuse="$INFO[colordiffuse]">texture.png</texture>
```

Works for all texture controls, for example textureradioon, textureradiooff, texturefocus, texturenofocus etc.