use a UI file

Hi Olivier,

this pull request aims at reducing a bit of the code by using a UI file for the dock.
the diff is not really easy to read, I believe PyCharm replaced tabs by spaces. Is this ok for you?

I removed your custom QLineEdit class and used an event filter, see beginning of CadInputWidget.py. This is working pretty well.

I also added icons, see:
![image](https://f.cloud.github.com/assets/127259/2065096/2aceaea6-8cdb-11e3-98bf-4928b1cac580.png)

Let me know what you think!
Cheers,
Denis