Code-completion for imports: pre-defined list

[fabioz]

Is your feature request related to a problem? Please describe.
RF does not have a way to say which Python libraries are actually RF libraries, thus, the extension only shows imports for libraries that were once imported (and for which the spec creation was successful).

Describe the solution you'd like
The extension should have a list of common libraries and try to pre-create a spec for those.

Describe alternatives you've considered
Another option could be having libraries themselves ship with the .libspec files (but that requires a change in the libraries packaging and is not really under the extension control).