Create library view

[nimakro]

We need to implement a library view, which will allow's to drag and drop items into the preview view or the hierarchy view.
The controls which are shipped with JavaFX are grouped as follows:

• Containers
• Controls
• Menu
• Miscellaneous
• Shapes
• Charts
• 3D
  I think for now we will keep the same categories as SceneBuilder, we can still change them in the future if needed.

And if custom libraries are added, they will also be shown in the library view as a separate entry.
In the future, we need to think about how external libs (controls) can be imported. In the first step, we only show the controls which are shipped with JavaFX and tornadoFX