Use empty paths for top level elements

This is a large refactoring of the ElementPath implementation that would lead to improved performance on Top level (corpus) browsing.

Needed for geosophy, balsac, any large project (1M+ element)

added Optimisation + 1 deleted label

assigned to @erwanrouchet

A benchmark on most corpora in dev did not show much of an enhancement, not even on Balsac with >1 million elements. It took an import of a corpus three times larger than the entire preprod (HOME Complete) to show a difference:

Value	1.2.0	!1591 (merged)
Total	>3600000	83.35
Average	>3600000	16.67
Total folder	4.43	4.65
Average folder	0.89	0.93

mentioned in merge request !1591 (merged)

mentioned in issue #943 (closed)

mentioned in merge request frontend!1222 (merged)

mentioned in commit 106dceff

closed