Xtext's transparency barely worked on windows, didn't work on any modern
linux wm and used fake transparency.
This uses gtk's built in window opacity that works on more systems and
is real transparency.
Text area only transparency may return with a transition to cairo, if it
works on Windows.
Text events allow %Cfg,bg as decribed in "Theming" on
https://hexchat.readthedocs.org/en/latest/appearance.html?#theming
That section describes mIRC colors 0-15 and local colors 16-31, but
when selecting background colors, the 9 special internal colors are
extremely useful, like %C20,33 will do bright red on the selected
text background color.
As the documentation is on-line, this patch has no doc-change.
Ideally, the color-codes should show in the colored squares in the
preferences color-dialog just as with colors 0-31
Signed-off-by: H.Merijn Brand <h.m.brand@xs4all.nl>