Adding QtDesigner custom widget plugin for KConfigXT

[Alexander Dymo]

> > It is really simple.  The idea was that the user would have the ability to set
> > all environment options in one place and all external processes of KDevelop
> > would use it.  Doesn't mean you still couldn't allow multi setups.
>
> IMHO thats unifying too much. We have multiple projects in KDevelop and
> thus some things will not work correctly when set in a
> global-env-widget, for example QTDIR above.

Yep, only per-project settings only make sense for me usually. But I
can imagine somebody working on KDE4-only projects and therefore in
the need of common env options. Still I find it quite a rare case. I
wouldn't like my ruby projects have the same env configuration as
kdevelop and kdevplatform. So what we have is ok for me and what we
might need is a "copy env settings" facility maybe.