Add descriptions/tooltips to properties in the Import dock

[Calinou]

See also #1339 and #1955.

Describe the project you are working on

The Godot editor 🙂

Describe the problem or limitation you are having in your project

Properties in the Import dock currently can't have any descriptions:

A lot of properties don't behave in an obvious manner (such as lossy compression quality for textures). This makes it important to document those properties in an easily accessible way.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

When hovering a property in the Import dock, display a tooltip with documentation.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Figure out a way to document import properties in the class reference by exposing them somehow. Maybe expose dummy classes for each imported resource type so they can be listed in the class reference?

If this enhancement will not be used often, can it be worked around with a few lines of script?

No, as this is core editor/documentation system functionality.

Is there a reason why this should be core and not an add-on in the asset library?

This is core editor/documentation system functionality.