WOrld Storage gizmos created for the use case.

# Conflicts:
#	Assets/ETSI.ARF/ARF World Storage API/Editor/Windows/TrackableWindow.cs
#	Assets/ETSI.ARF/ARF World Storage API/Editor/Windows/WorldAnchorWindow.cs
#	Assets/ETSI.ARF/ARF World Storage API/Editor/Windows/WorldLinkWindow.cs
#	Assets/ETSI.ARF/ARF World Storage API/Editor/Windows/WorldStorageWindow.cs
#	Assets/ETSI.ARF/ARF World Storage API/Materials.meta
#	UserSettings/Layouts/default-2021.dwlt
#	UserSettings/Search.settings

Create button now in the main window.
Different colors and icons for World Elements.

Draw gizmo for WorldLinks.
From/To link endpoints can now directly be selected in the scene.

Group concept as KeyValue.
Visual for trackables, anchors and link via prefabs.
Edit link now run well (TODO: drag/select elements?).
Code optimization in WorldLink.
TMP Pro for UI.

Main window: filtering by value for key "Group".

- added a to/from request button (tbi)

Some bugfixes.

Major structural and design changes on GUI, classes, parameters...

See merge request !2

Added world link window.
Completion and implementation of all API parameters (tbd: to be tested).

Feature/with submodule

See merge request !1

Changed some folder, script names, namespace...

Minor GUI changes (layout, Name of element)

Renamed the server scriptables.