Get rid of all the limitations in .ui.qml files

There are some limitations imposed on the user when using .ui.qml files, such as Behavior is not usable and JavaScript blocks are not allowed, yet, they are the only .qml files which the users can live preview.

These limitations hinder productivity and, what is worse, annoy uers badly. We should get rid of all of these limitations and let users decide how they perceive the role of .qml and .ui.qml files, in that they can write javascript blocks and use Behavior inside .ui.qml if they want, etc.

All of these limitations were imposed just because of how QDS works, which is none of the users' business. They can write perfect QML code just using Qt Creator and .qml but because of these limitations they need to create extra combination of .qml and .ui.qml files and this is not right.