Adding QtDesigner custom widget plugin for KConfigXT

[dukju ahn]

I'd like to port kdev3's environment variable widgets into
current makebuilder project setting page. Currently it uses KEditListBox.

I found that to be used with kconfig compiler, custom widget should
have name "kcfg_name" in qtdesigner file. That means, we have
no way but to add custom widget plugin for designer. kdelibs
already installs its own designer plugins.

Is it allowed thing that kdevelop installs widget plugin for
qt designer? Or is there any simpler approach that custom widgets
are compatible with KConfigXT? I want to check before I start.