I added a `showHeader` option to the `item` builde...

# tornadofx

[edvin]

I added a showHeader option to the item builder for drawer. It will default to true when multiselect is on, otherwise false. The reasoning behind that is that you'd need/want the title header to distinguish between the different content in the content area. You can control it completely manual too of course, but by default it does the right thing with no extra code.