Custom Property Extension: String Validation

Nobody has claimed this yet.

**Feature**

We have started using Tiled as our all-in-one map builder for a game in my custom engine. The long-story-short is that we have decided to attach dialog to characters on the map per character occurrence, and need a way to validate the text for the writer.

**Proposed Solution**

Because this is potentially a niche use-case, I think the broader solution is the ability to add an extension that creates a custom-named string property type, which can be validated by the extension. Even just turning the box outline red and showing some text when invalid is enough for this specific use-case.

**Alternatives Considered**

I made an external tool that does exactly this basic functionality, but getting the writer to cross-reference an associated dialog ID in Tiled to the tool is proving difficult. If they could find the character on a map and edit a property of a custom type "Dialog" to get immediate feedback, that would make Tiled a perfect solution for our use-case.

**Notes**

I'd be willing to contribute towards this if I can get some feedback on whether this could be approved, and would be useful to others.

No files, tests, or entry points are named. Start by locating Tiled's custom property and extension systems, then determine how a custom-named string type could validate values and report errors in the editor. Done means an extension can define the property type and provide visible validation feedback while editing.

Written by the indexing model from the issue text.