layouting improvements

- layout mode deactivation area extended to cover whole widget area's
header
- widgets switchable without layout mode by header drag now
- tab pane improved widget switching & selection
- much simplified custom content dragging
- removed or refactored useless classes
configuration api improvements

- more robust api
- inherited fields now work too
- PropertyConfigs now work for non static fields
- StaticFieldConfig and InstanceFieldConfig classes merged
- Configurable & Configuration more unified now
- several code duplications removed
- additional enhancements