This now allows text view to render text with alpha values in
the text foreground and backgrounds, the work is almost complete,
currently the error-underline-color is still a GdkColor style property
and since we use only GdkRGBA for rendering it needs to be converted
and applied, probably a new rgba version of the style property should
also be introduced.
This commit adds tests/testtextview that must be run from the tests/
directory to show translucent text in action.
