Aseprite Importer: move importers out of the inspector

[stickgrinder]

Benefit description

The Aseprite importers will belong in popups or panels instead of cluttering the inspector.
The current positioning of that interface comes from mimicking PowerQuest placement, which comes from specificities of Unity and - although sensible - is not the best option.

Solution description

We still need to decide where to place the importers. @stickgrinder will complete this issue as soon as we design the new interaction.

Exclusions

None

Implications

To achieve this goal, the Importers will be changed from an Inspector Plugin to an Editor Plugin (like the Gizmos one introduced in #232).

This new architecture, based on a master and many subsidiary plugins, works pretty well to keep things isolated and maintainable. Still, it will require us to decide on an overarching set of standards for how those plugins will interoperate.

I'll leave some comments with options we can see at the moment.