Check to detect changed read-only string

### Describe the problem

When a string is marked read-only, it can have translations, which are no longer editable. Upon source string change, Weblate would mark translations as needing editing, but it can not because the string is read-only. So, there is no way to indicate this to a user to deal with this.

### Describe the solution you'd like

Add check to indicate this has happened:

* Upon source string change on read-only string, check would be triggered
* The actual check code be no-op, it will always stay triggered until manually dismissed

### Describe alternatives you've considered

Automatically removing `read-only` flag upon source string change. This would allow editing of the changed string, but can go unnoticed.

### Screenshots

_No response_

### Additional context

_No response_