Fix tooltips behaving incorrectly on Tree nodes

[YeldhamDev]

Tooltips within Tree nodes behave awkwardly on Godot 4. Normal ones don't disappear when switching to another item, and custom ones don't even update at all. This PR fixes both cases.

[YuriSizov]

A think this makes sense. There is definitely a visible usability improvement, the only question is whether this may lead to some regressions and excessive updates for some tooltip. But I couldn't notice anything from a quick test with the editor.

Old

godot.windows.editor.dev.x86_64_2023-09-28_13-12-01.mp4

New

godot.windows.editor.dev.x86_64_2023-09-28_13-17-32.mp4

[YeldhamDev]

@YuriSizov Well, the way it works is still the same, string comparison. The only difference is that now it's a little more sophisticated.

[YuriSizov]

I'm not so worried about string comparison performance. What I meant is that this changes when we request an update for the tooltip itself. If this somehow could lead to more updates to it than previously and than needed, this would be a performance problem. But we'll see.

[YeldhamDev]

From my tests, it only triggers updates when the tooltip string itself changes, which is what we want to do.

[YuriSizov]

Thanks!