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

[backend][assetmodel] Made AssetType a python enum

See merge request !59

This will make the enum itself more usable. Style wise, this makes
it an independent element of AssetModel.

Reusable widget short description

See merge request !55

Fix settings placement after components creation

See merge request !56

refactored save/restore state/geometry with functions
renamed functions to save/load settings