[4.3] Added get_open_scenes_roots to Editor Interface API

[Spartan322]

See godotengine/godot-proposals#10895
Closes #907

(cherry picked from commit godotengine/godot@5d72926)

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Original get_open_scenes function provided by Godot is quite limited as it only returns a string array containing the names of opened scenes. This feature is necessary for situations where accessing all opened scenes in depth is required and it is also essential for the Jenova Framework Hot-Reloading feature.

This commit includes :

• Changes to the Editor Interface API (editor_interface.h and editor_interface.cpp)
• Updates to the Editor Interface documentation (EditorInterface.xml)

[Spartan322]

Will consider cherry-picking to master if 4.4 lapses in including this by the time of Godot 4.4 feature-freeze.