Add support for styles in GUI

[mrDIMAS]

fyrox-ui still does not have an easy way of widget styling. It is possible to customize pretty much any widget, but it is very tedious and extremely complex for simple cases like changing color scheme. I suggest to add style tables, that will contain a set of properties for every widget. Style table should support inheritance to make it easier to create derived styles with small changes over base style.

This task seems to be quite large, since fyrox-ui contains 40+ widgets and each one requires individual style table.