Add a note that 'setWidth' sets a width to a 'flow-component-renderer', not an overlay/dialog itself

[anasmi]

Dialog dialog = new Dialog();
dialog.add(new Label("Close me with the esc-key or an outside click"));
dialog.setWidth("80px");

Sets the width of the div inside the flow-component-renderer, but not the dialog itself

This (as turned out )is in line with the acceptance criteria of a setWidth #35 (comment)

Though, it would be nice to add a mention about this behavior to JavaDocs, since it's not what a user expects by default.

[web-padawan]

Fixed by #147 and will be included to 2.1 minor release as a behavior altering fix.