Fix has_filter of AnimationNode not being called in scripts

[olibombardier]

In godot 3.5, The has_filter() method was not called when implemented in a custom AnimationNode, making it impossible to have the "Edit Filter" button show up in the editor.

[Calinou]

Is this issue also present in 4.0.alpha? If so, please open a pull request against master too 🙂

[olibombardier]

I did a bit of testing and the issue is not on the 4.0.alpha

[michael-nischt]

As pointed by @Hiiamwilliam, the binding is wrong too: (falsely returns string instead of bool)

If it's quicker to also add it to this PR for getting it the 3.5 branch, please do so.
Otherwise, I kindly ask to please merge this one instead. 🙂

[olibombardier]

I didn't catch that, I fixed it here too just in case. That reminds me that the doc also says that the methds should return a string. I'll open a PR to correct that too.

[Calinou]

If it's quicker to also add it to this PR for getting it the 3.5 branch, please do so.

Pull requests for the stable branch should be merged on 3.x first, as bug fixes can be cherry-picked later to 3.5. Do not open pull requests directly against the 3.5 branch.

Make sure the binding is updated as @michael-nischt described, then we can merge this in 3.x once checks have passed.

[olibombardier]

Make sure the binding is updated as michael-nischt described, then we can merge this in 3.x once checks have passed.

The binding is corrected and I made sure the checks passed on my fork

[YuriSizov]

Seems to be indeed fixed in master with the GDVIRTUAL change. Thanks!

[akien-mga]

Cherry-picked for 3.5.1.