Add the possibility of default selection to MultiItemSelector

[Drevsh]

I hope I did everything correctly. (It's my first time committing to an open source project)

I think there might be a problem regarding default selection in the SingleItemSelector.
You can now set multiple items as selected in the SingleItemSelector and the DefaultSingleItemSelectorContext will return one at random.

		@Override
		public Optional<I> getResultItem() {
			if (getResultItems() == null) {
				return Optional.empty();
			}
			return getResultItems().stream().findFirst();
		}

Or that is at least what I think will happen. Maybe this needs to be handled differently in SingleItemSelector/MultiItemSelector

[jvalkeal]

Thx, it looks something how I'd done the quick hack to add this feature without rewriting shared parent class with single/multi. It probably gives some bullets for a user to shoot own leg if this is used with single selector but I think we can address that later with additional changes or documentation.

I left some comments to address. Feel free to add yourself as author to files if you want. I think we can sneak this in into 2.1.0.

[Drevsh]

Hey @jvalkeal I hope I got everything.

[jvalkeal]

Thx a lot, merged per 2862cc8