Delete BlTransparentPaint

[tinchodias]

This is a subclass of BlColorPaint, that only overrides a couple of methods such as isOpaque, and implements the Singleton pattern in class side. But we can have that singleton directly as BlColorPaint transparent. No need/visible benefits of a special subclass.

[tinchodias]

More comments: there is a BlTransparentBackground that is a kind of NULL background that makes this Singleton not useful. But there is a BlBorder class>>empty that (IIUC) uses this paint's singleton. However, that empty border is also cached in a BlDefaultVisuals' class variable so again another possible use of the BlTransparentPaint is discarded.

However, in the PR a just converted the transparent paint as a sirect subclass of BlPaint. This way is cleaner.