Read-only

1. Change a content in the editor.
2. Turn on read-only mode by clicking the button above the editor.
3. Check if undo/redo (by shortcuts) is disabled.
4. Check if modifying content is blocked (input, delete, paste) for collapsed and non-collapsed selection.
5. Check if toolbar buttons are disabled.
6. Check if toolbar buttons reflect selection attributes (eg. when selection is in bold text then bold button should be on).
7. Check if link balloon panel opens in read-only mode.
8. Check if image text alternative balloon opens in read-only mode.