enh: much better fileComponent keyboard experience

[SystemChanger]

When there is selected component (by resizing controls):

1. Typing text keys (e.g. A-Z, 0-9) now replaces the selected component
   Logic there is the same as for text nodes - it acts like if the component/image is a selected range of text. Unification.
2. Enter inserts a new line after the selected component and sets the carriage position on it (corresponding to the behavior of the insert line below button)
3. Shift+Enter inserts a new line before the selected component and sets the carriage position on it (corresponding to the behavior of the insert line before button)

@JiHong88, try it. It is pretty nice and intuitive right now! This is the behavior I was expecting by default.
P.S. I hope my way to remove/destroy the container is sufficient (on typing text keys): util.changeElement(container, format);
New behavior also prevents some nasty bugs and oddities on typing text when resize controls are active. Try it with other PR's altogether.