Tooltips for the Import dock

[vnen]

Operating system or device, Godot version, GPU Model and driver (if graphics related):
Godot 3.0 beta

Issue description:
The import dock could have tooltips for the options with descriptions, to help the user understand what they do. Currently it only shows the internal name. This should also be exposed to custom import plugins.

[sonic2kk]

Was this fixed by the commit referenced in #11186?

[akien-mga]

No, the Import dock does have tooltips as it uses the same Inspector widget, but the import options are not exposed to the documentation and therefore there's no content to put in the tooltip yet.

[Calinou]

Closing in favor of godotengine/godot-proposals#2041, as feature proposals are now tracked on the Godot proposals repository.