Allow users to edit custom libraries directly in the editor

[m-mohamedin]

Currently users have to download, edit, re-upload.

Requested for conf cloud, but applies to core.

[mararad]

"Allow library shapes to be edited after creation. Double clicking could open a new sheet to edit that component. Saving updates the library.
Instances of library components could update the drawing when they are updated. This could be optional. Other tools generally can't do this, which is a big drawback for technical documentation, so doing this would be a big draw.io plus point"

https://trello.com/c/fEkZ1Gf0/44-allow-library-shapes-to-be-edited

Applies to core also