Fix method dialog label

[KoBeWi]

In the method pick dialog, the label that appears when no methods are found could be at wrong position (probably due to being child of Tree).

Also if you selected an empty script, the label did not appear due to empty root item:

With this PR the label is centered in the dialog and the "Attached Script" item only appears if there are any script methods available to pick.

[YuriSizov]

This is probably rooted in something unrelated, but because the label is directly parented to a Window node, it doesn't always work. If you open a dialog where it is supposed to be immediately visible, you can see it. But if you open a dialog where there are valid methods, and then filter it, it does not appear. This is repeatable, and you can try it with either case in any order, and it will still happen.

It is also possible to "cure" the editor and make the label appear correctly until you restart, if you close the dialog in a state where the label is supposed to be visible. Like, if you filter the dialog so it is supposed to appear (and it doesn't), then close it and open it again, the label will start appearing correctly after that.

Edit: And it breaks again if you try to connect a signal that has no matching methods.

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Seems to be some desync issue with the visibility propagation when we try to immediately hide it, I think. Still looking into it, but just wanted to leave a comment so we don't merge this in the meantime and get unexpected reports.

[YuriSizov]

I think this is related to the fact that we never update parent's visibility for windows when the node itself is hidden. Because this is explicitly disabled in CanvasItem::_window_visibility_changed(). I'll try removing the check, it should be safe.

Edit: Done #75890.

[YuriSizov]

Aside from the issue with the window, this seems to work as expected and I don't see any issues with the layout or input propagation from parenting the label directly to the window.

[YuriSizov]

Thanks!

[YuriSizov]

Cherry-picked for 4.0.3.