Synchronize screen widgets

Signed-off-by: Irmo van den Berge <bergerkiller@gmail.com>

There was a problem with the onTick method being called by the scheduler, and at the same time the render thread altering a widget (The debug screen GUI, which dynamically changes the amount of widgets for lines)

I do not particularly like to use synchronize blocks, but I saw no better alternative. Since widgets shouldn't change THAT often, it should be sane in this instance.