Changed widget container from frame to dockwidget.

Backend preview, standard output and finite state machine graph widgets
are now displayed inside a dock widget.
Further support for ROS launch file and several refactoring.

- Refactored TreeItems.
- Added Item Menu entries for adding roslaunch files.
Added further functionality for ROS launch files.

- ROS launch file items now support global parameters, extra includes
and nodes to be launched.
- minor bugfixes
Added Icons.

- Added Icons for ROS launch file and RoCon launch file.