Replace Composite for SourceTypeSelector/TitledAttachmentFileWidget

SourceTypeSelector and TitledAttachmentFileWidget extends Composite, while we are currently strive to move toward new Elemental. At the same time, the list where they are using is expecting a IsWidget type. So, try if already a similar widget exists inside appformer/kie, otherwise create new one and (eventually) implement the asWidget method and returns a "Widget".
Later, ask Rikkola to move it on upper framework.

ADDITIONAL IMPROVEMENTS:

1. When there are two .dmn files in the project, the second one is not in the list when creating a new test scenario asset. The list must be refreshed to contain all available .dmn files. The same problem occurs also with a single .dmn file in the project. Sometimes the list of available .dmn files is not refreshed and even though they are in the project, they are not displayed in the list. -> Addressed in !1047.
2. Change Choose DMN asset to Choose a DMN asset.
3. The box for choosing a .dmn file is not wide enough. Make it either as wide as the boxes above (for the name of the asset and the package) or change it to a drop down (the same style as the package drop down).
4. The value of a .dmn file should not be pre-filled once the box has been used. Clear the status so the box is always empty. -> Addressed in !1047.
5. The field Choose DMN asset should be marked with * (required field). -> Addressed in !1047.