Tree: Calling update in _gui_input less frequently

[giulianob]

Optimizes _gui_input handling in tree control to only call update() when going from a control that actually needs to re-render which are the ones that use hover states (columns and cells which have set_custom_as_button set to true).

1. Columns already had a condition to only update when they changed but there was still an explicit call to update() which can safely be removed.
2. Calling update() only when hovering in/out of cells that use set_custom_as_button.

Can this fix be cherry picked for 3.2.2 since it's backwards compatible?

Also, I'm not sure if anyone out there is using set_custom_as_button it looks like the editor used it a few years ago but no longer does. It's barely documented and I had to poke around the source code to figure out how to even get it to work so I'd be surprised if anyone is using it. Might be something good to remove for 4.0 if it's a good time to deprecate unused APIs.

Test project: TreeSample.zip

Still seeing some updates that could be optimized but much less than before. The main benefit here is when I'm moving my mouse around the first 3 groups of cells which don't have any hover state, there are no draw call updates. Everything else is still working as expected.

Fix: #39251

[akien-mga]

Thanks! And congrats for your first merged PR (and a well detailed/tested one :)).

[akien-mga]

Also, I'm not sure if anyone out there is using set_custom_as_button it looks like the editor used it a few years ago but no longer does. It's barely documented and I had to poke around the source code to figure out how to even get it to work so I'd be surprised if anyone is using it. Might be something good to remove for 4.0 if it's a good time to deprecate unused APIs.

Yeah I think that could be a good candidate for removal in 4.0.

[akien-mga]

Cherry-picked for 3.2.2.