Gtk improvements

As already announced, I worked on some improvements on the GTK UI.
Hightlights of thsi pull request:

1. New About dialog
2. Use desktop icon also as application icon
3. Target selection dialog
4. Filterable connection list
5. Rework of Connection Detail and Profile Detail dialog
6. UI copes with asyncronous name resolution
7. Get rid of gtkui_details_window() and gtkui_details_print() function

There are some more minor changes and some fixes.