Added GdkRGBA properties to GtkTextTag.

Added GdkRGBA properties to GtkTextTag.

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.