add SolidBrush, LinearGradientBrush, TextureBrush

[pyzon]

Hi,

I've made some improvements to the Brush class. I needed the linear gradient and the texture brush for my project, so I made the Brush class abstract and derived the concrete brushes (inlcluding the solid brush) from it. Regarding the new brushes, I did not implement all the functionalities, but it's a good starting point.