Apply new input validation method for Create Plugin dialog

[eugene87222]

Fix #76681

• Add status panel to the create plugin dialog
• Remove unused info icons (for old validation)

Old	New

[KoBeWi]

I left some style comments that need to be addressed, but otherwise looks good.

I think given how this validation convention is widely used now, it might be worth it to create a dedicated class to handle it. I have some idea, but it's for another PR.

[YuriSizov]

Is this an overall improvement though? We lose direct association between the error (or successful validation) and the field.

[KoBeWi]

Well, you could say the same about every dialog where we use this type of validation.

[YuriSizov]

Yes, you could. Which makes me skeptical about it in general. In some more complex cases it kind of makes sense. But here it's a simple form, and we already have nice validation, at least as far as immediate hints from icons are concerned. And we want to remove that for some text that you need to read and figure out and associate back with the form.

[KoBeWi]

#78744 will unify all these validations into a single class, which will allow us to easily modify it. Then we can e.g. associate specific errors with some fields.

[KoBeWi]

#78744 was merged, so this needs to be updated. It will simplify the code.

[KoBeWi]

The "subfolder valid" message still appears when you edit the plugin, even though the path field is not visible. I think the path message should not be displayed when editing.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉