Fix autocompletion for elements in sub-folders

Description

As described in #448 (closed), autocompletion may resolve element path outside of the element-path directory configured in project.conf, and this is not intended.

Changes proposed in this merge request:

• Fix the bug in complete_path(), from _frontend/complete.py.
• Add a test for that specific use-case.

This merge request, when approved, will close issue/bug: #448 (closed)