Added support for custom button background image.
Added support for separate button title font color.
Button title font size is set to fixed and separate value from title font size.

Currently button height is fixed to 31 px. When title font size was set to bigger value like 16 button was looking bad.
Added support for separate content text color.
Added missing values in design.json.
Added support for separate button title shadow color.
Added support for separate button title shadow offset values.