Antialiasing and diagram properties dialog fixes

On my system (Slackware 14.1) the diagrams look really ugly in 0.018; see in the enclosed pictures the difference between no aliasing in 0.017, the 0.0.18 aliasing and 0.0.19 after fixes, especially around the log graph lines and around the curved parts of graph lines (need to see at 1:1 scale of course). I saw in the code that now antialising is enabled by default; googling around I saw this seems to be a known issue with Qt4 ( http://qt-project.org/forums/viewthread/28093 , http://lists.qt-project.org/pipermail/development/2012-October/007033.html ) . One of the possible fixes seems to keep the drawing coordinates as floating point, instead of rounding them to int as currently done in Qucs, so I modified viewpainter.cpp accordingly. Note that to be consistent QPointF should be used also elsewhere, but for the moment things works nicely anyhow.
![qucs_0 0 17_no_antialiasing](https://cloud.githubusercontent.com/assets/9018179/4783056/977b08e0-5d16-11e4-9ca9-0a3a3f084a66.png)
![qucs_0 0 18_antialiasing](https://cloud.githubusercontent.com/assets/9018179/4783057/9eb36986-5d16-11e4-8bae-d1aedf81b7ac.png)
![qucs_0 0 19_patched_antialiasing](https://cloud.githubusercontent.com/assets/9018179/4783058/a3720a68-5d16-11e4-87bb-2515c76c6a29.png)
IMHO, there should be an option somewhere to disable antialiasing, might add this later.

I have also modified the Edit Diagram dialog to have it look more similar to 0.0.17 and to have the data names sorted as they were before, see enclosed pictures.
![qucs_0 0 17_edit_diagram](https://cloud.githubusercontent.com/assets/9018179/4783069/58f5c460-5d17-11e4-905d-4be86a61f428.png)
![qucs_0 0 18_edit_diagram](https://cloud.githubusercontent.com/assets/9018179/4783071/5e369b84-5d17-11e4-9e2b-1af6630444d0.png)
![qucs_0 0 19_patched_edit_diagram](https://cloud.githubusercontent.com/assets/9018179/4783072/62b96114-5d17-11e4-97fb-efad8768de0c.png)

Finally I have also slightly modified the main configure.ac to remove duplicate search paths for Qt headers and added the default path used in Slackware 14.1 (otherwise ./configure will fail)