Rethink/redesign the different browsers

Qt Design Studio has currently multiple browsers.

Components, Assets, Materials, Textures and Content Library.
Currently this is very implementation driven and there are many idiosyncraties.

Components contains simple components like Rectangle but also complete templates for e.g. particles.

3D Models for examples are QML components and cover several files. Therefore we keep them in Components, while users might expect them in assets.

Assets contains single asset files, while the texture and material browser contain the definitions of textures/materials inside the document, despite the fact they could be considered assets, too.

The Content Library contains items that can be downloaded and added to the project.

All of this has to be reconsidered and maybe the result is a single browser that can handle all these use cases and future use cases.