External tool can modify read-only editors

Steps to reproduce:

• open a ui file and switch to Edit mode
• Creator states this file can't be edited - editor is set to read-only
• mark some lines
• invoke Tools -> External -> Text -> Sort Selection

Current result:

• content of editor is updated (sorted)

Expected result:

• content stays the same (or the Tools that might change the editor's content aren't even enabled to trigger them)