Introduce WrapLayout to replace FlowLayout

FlowLayout does not really flow components into separate lines when they
overrun their horizontal space. WrapLayout does this.

Also moved the contents of org.simantics.utils.ui.awt into
org.simantics.utils.ui.internal.awt since that code was never meant for
public use. Moved WrapLayout into org.simantics.utils.ui.awt and
published that package.

refs #6814

Change-Id: I65600f9a5edff021ab011d980bb3352f6141177f