Scopes of scratches

[sxhya]

Select any piece of text and use Create new scratch file from selection intention to create a separate .ard-file outside of Arend project (this is a standard Intellij feature)

Currently every such scratch displays message that it is outside of sources folder.

1. A reasonable implementation, IMO is to link the created scratch to the project where it was created and use project's src directory as (implicit, non-configurable) src folder for the scratch.
2. Modify default behavior of this intention in such a manner that it would create \import-header in the destination scratch for all links mentioned in the migrated piece of text.