Preserve state of the terminal

[mvysny]

Removing XTerm component from the parent container then adding it back will result in a blank uninitialized terminal: there is no scrollback history, and the settings (such as cursor style or similar) are not preserved.

The code which reproduces the issue:

public class MainView extends VerticalLayout {

    public MainView() {
        final XTerm xterm = new XTerm();
        add(xterm);
        xterm.setWidthFull();
        xterm.setHeight("500px");
        xterm.writeln("Hello!");
        xterm.setCursorStyle(ITerminalOptions.CursorStyle.UNDERLINE);
        xterm.setCursorBlink(true);
        xterm.write("$ ");
        xterm.addLineListener(e -> {
            System.out.println(e.getLine());
            xterm.writeln("You typed: " + e.getLine());
            xterm.write("$ ");
        });

        final Checkbox checkbox = new Checkbox("Add/remove xterm");
        checkbox.setValue(true);
        add(checkbox);
        checkbox.addValueChangeListener(e -> {
            if (e.getValue()) {
                MainView.this.add(xterm);
            } else {
                MainView.this.remove(xterm);
            }
        });
        add(checkbox);
    }
}

Steps to reproduce:

1. Type in something
2. Uncheck the checkbox
3. Check the checkbox back
4. Observe that the xterm settings are not preserved.

Just a wild suggestion: perhaps the settings and the scrollback buffer could actually be preserved on the Java side and applied every time the component is attached.