Test cleanup

Library editor

See merge request !63

Fixes #178

Core implementation

See merge request !62

DescriptionWidget was over-engineering.

This decorator disable the possibility to add attributes to
to python objects outside the __init__ function.

Now:

- Only shows know assets subfolder
- Only show the version of the asset to edit

Part of #186
Replaces !51

The AssetWidget class shows the correct editor for the JSON
data to be loaded.

It also handles the saving part.

Part of #184

Part of #182

Part of #181

Part of #178

Part of #185

Part of #179

Part of #183

Part of #180

This is a "do nothing" editor that provides the editor API.

To be used as placeholder.

- Add description widget as it is common to all editors
- Made load/dump_json method concreted that will merge subclass
  json data with common json data
- Add pure virtual _load/_dump_json protected members
- Implement dirty flag handling

Also made the enum unique so that there's no way to have two
same values in it.

Add library re-usable component

See merge request !60