Standardize dialog input validation as a new class

[KoBeWi]

Since some time we have an established way to validate input in various dialogs. We use a panel with error messages, like this one:

However every dialog implemented it separately, with led to duplicate code and even some inconsistencies (like missing panel style or using wrong listing character).

This PR adds EditorValidationPanel. It serves 2 purposes:

• displays provided messages, styling them depending on message type (ok/warning/error/info)
• reduces boilerplate code by doing some error management automatically

EditorValidationPanel's constructor expects a vector of strings. They will serve as default "valid" messages (i.e. no errors); size of the vector determines number of labels. After panel is created, use EditorValidationPanel.update() to queue an update method. This method will first setup labels to their default state (so the status is "valid" by default and you don't need to set it each time), then a designated update callback is called. In this callback the user is supposed to use set_message() method to set the texts of the labels. The set_message() takes label index, message text and message type (ok/warning/error/info). EditorValidationPanel will automatically prefix the messages with •, assign them colors based on type and disable or enable the dialog's OK button if it was registered.

I replaced all custom validations with EditorValidationPanel, in some cases making them more consistent (although I tried to preserve the previous appearance, notably the notes without bullet points in script create dialog). This also exposed some messages that previously weren't previously used due to faulty logic. I noticed that AnimationBlendSpaceEditor also uses some similar panel with errors, but it behaves a bit differently and is not a dialog, so I left it for now (although the validation panel is capable of handling it).

EDIT:
After latest changes the messages are no longer part of the constructor. Instead you need to define them with explicit ID.

EDIT2:
Fixes #80218

[YuriSizov]

Implementation looks sensible now. I understand your arguments and they make sense to me. I've tested all 4 dialogs a bit, and it seems to work as expected, though admittedly I didn't test every error message.

One thing that caught my eye, which isn't related to this PR, but can be addressed by it, is that the template message is colored like a successful check, when it should probably be an info message:

(Also apparently this is a valid filename, who knew!)

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There are some small suggestions (and a typo), but LGTM otherwise.

[YuriSizov]

This could be _update_theme_item_cache, though in such a small class it doesn't change much.

[akien-mga]

Thanks!