QGIS custom widgets plugins for Qt Designer

This adds a new library for using custom widgets in Qt Designer.

I implemented as a first try a a group box for scale dependent visibility.

I successfully built this on ubuntu, but did not try on Win/Mac.

@jef-n may you have a look at the CMakeList files?
@timlinux would be nice to have your opinon since I believe you were waiting for such feature ;) Also, could you have a look if it works well on mac?