NOTIFICATION_THEME_CHANGED is getting called when it shouldn't

[AaronRecord]

Godot version

883bb2f

System information

Windows 10

Issue description

See https://chat.godotengine.org/channel/devel?msg=J4SjAHxFmwZ7NHYqQ

When opening a project in 3.x, NOTIFICATION_THEME_CHANGED consistently gets called 8 times (or at least on the FileSystemDock and ProjectExportDialog which were what I tested. In master it's only 3 times).

When running the project manager in 3.x, NOTIFICATION_THEME_CHANGED is only called 3 times in EditorAbout (although it doesn't get called at all in master because (maybe by mistake?) NOTIFICATION_THEME_CHANGED was removed from EditorAbout in master).

NOTIFICATION_THEME_CHANGED is also called when/right before an object is destroyed it seems (or at least when closing the editor.

In master NOTIFICATION_THEME_CHANGED is only called three times.

Steps to reproduce

Insert a breakpoint in NOTIFICATION_THEME_CHANGED

Minimal reproduction project

No response

[AaronRecord]

So the reason it's getting called 3 times is

1. For case NOTIFICATION_ENTER_TREE
2. NOTIFICATION_THEME_CHANGED gets called in NOTIFICATION_ENTER_TREE
3. NOTIFICATION_THEME_CHANGE gets propagated to all children in add_child_notify. Note this doesn't get called if the control's parent isn't also a control.

_propagate_theme is also used to set the theme_owner to nullptr in remove_child_notify, but there's no check in _propagate_theme to see if the control is being removed or added, which is why NOTIFICATION_THEME_CHANGED also gets called when the control is removed. Calling NOTIFICATION_THEME_CHANGED only really needs to be done when a control node is entering the scene tree, as none of the theme changes will be visible when the node is outside of the tree.

[AaronRecord]

I understand why NOTIFICATION_THEME_CHANGED is called so 3 times in master, but I haven't looked into why it's getting called so much more often in 3.x (8 times instead of 3). For whatever reason the control's theme is emitting changed several times, calling _theme_changed, which is what is causing all the extra notifications.