UIElement handling
- Each paragraph should have UIElement at it's bottom showing "END OF PARAGRAPH".
- UIElement should not block typing or prevent regular editor usage.
- When paragraph is split or new paragraph is created - new UIElement should be created too.
- You should not be able to place selection inside ui element or type in it.
- Arrow keys should work correctly around ui element.
Note: You can't put a selection after X (UIElement).
See a comment in issue #1592.