Add default scenario directory in wizard

* Check if the file exits. If it does, then doesn't allow rewrite this
* file.
Add Content-type for PerfCake scenario

* PerfCake scenario should be opened in scenario desginer editor
Do not set preferred size of content pane

* The preferred size of content pane in PerfCakeTwoPartRectangle used to
* be set to maximum width, which was allowed by the parent figure. Due
* to this intervention to layout manager it stopped resizing the figure
* automatically so that it needed to be implemented manually. Now it
* should be done by layout manager itself.
Fix default constraints for scenario layout manager