removed duplicates from Qt headers search paths and added path used by Slackware
make dataset variables table more similar to 0.0.17 (sorted names, rows not numbered, properly named headers)
used float coordinates (almost) everywhere as a workaround for Qt aliasing issues (see e.g. http://qt-project.org/forums/viewthread/28093). Removed also some Qt3 support members in the process.
Merge branch 'master' of https://github.com/in3otd/qucs into in3otd-master

Conflicts:
	qucs/qucs/diagrams/diagramdialog.cpp