Focus Script Editor text on dock focus and click

[kitbdev]

What problem(s) does this PR solve?

• Fixes [4.8 dev4 regression] Using Open Script Editor shortcut does not focus the text area of the script editor #122908
• Supersedes Make script editor gain focus when button is pressed #122910 (as requested in rc) cc @Goldenlion5648
• Supersedes Allow script tab shortcut to refocus script text input #121203

This makes many actions focus on the Script Editor/Shader Editor text. Including clicking on the Script / Shader Editor tab, using the shortcut (including when its already open), using the Editor Dock menu option, restoring after floating, and using EditorInterface.set_main_screen_editor("Script").
Also applies when they are floating.
These either match 4.7.2 behavior or shouldn't cause any issues.

Additional information

Adds a callback when a dock is focused (unexposed). The Script Editor uses this to move the focus to its CodeEdit. Also applies to Shader Editor.
When a tab is clicked it now calls focus_dock. I don't think this has an effect in most cases since the tab would already be focused.

Also moved ScriptEditor::update_layout since it was near the unrelated ScriptEditorQuickOpen methods.

[kitbdev]

• Rebased after Properly deprecate _has_main_screen() method #123604

Now uses force_focus_dock.
Fixes an issue where the ScriptEditor wasn't being switched to when on the game/asset store tabs for various actions.

[KoBeWi]

Adds a callback when a dock is focused

Isn't NOTIFICATION_VISIBLITY_CHANGED enough? That's the intended way to perform actions when a dock is focused (made visible).

[kitbdev]

Isn't NOTIFICATION_VISIBLITY_CHANGED enough? That's the intended way to perform actions when a dock is focused (made visible).

It's not enough, especially for shader editor which needs to be able to open without taking any focus when clicking on nodes.
Also it should be able to focus even when it is already open.