Extract EditorMainScreen from EditorNode

[kitbdev]

• related Extract BottomPanel from EditorNode #87760

Extracts all functionality related to the main screen from EditorNode.
Behavior should not change.

Details

In _get_main_scene_state(), removed the main_tab key. I don't think it has been used since 081a236.
In _set_main_scene_state(), removed the editor_index key check. Only used between 334a818 and #11075 (less than a month during 3.0 dev).
The state in only actually saved and used in _save_central_editor_layout_to_config() and _load_central_editor_layout_from_config(). _get/set_main_scene_state() is just used for switching between 2D/3D when opening a scene or changing selection to match the node.

Removed distraction_free->move_to_front(); from EditorNode::add_editor_plugin since it isn't needed. The distraction free button is already after everything else in the SceneTabs, and if it was needed it should be somewhere else.

set_visible_editor() and editor_select() did the same thing. They are now EditorMainScreen::select().
Renamed editor_plugin_screen -> selected_plugin, and removed some other editor_main prefixes.

The scene_root_parent is now the editor_main_screen. It is not actually the parent of the scene root, and this way I don't need to add a new control to hold the vbox.
I could make the editor_main_screen actually be the vbox instead of being parent to it, and the scene_root_parent can be called main_screen_parent, if that is better.

Also removed unused forward declarations in EditorNode.

[kitbdev]

Fixed crash, it was because I used get_plugin_index which can return -1 but I didn't check for it. Also added ERR_FAIL_INDEX_V so it won't crash if it happens again.

Only plugins with a main screen are now added as children to the main screen control, otherwise they use gui_base.

[KoBeWi]

Looks fine.
I noticed some potential improvements that could be done in a follow-up, e.g. merging buttons and editor_table by using a struct.

[akien-mga]

Thanks!