Default slider position

Adds DSL method to set slider to a default position.
Falls back to the value of the field or the average value between start and end.

References #81091480

Squashed commit of the following:

commit aa0e10071a3017f68f5a358e948d98e49d98129b
Author: Ivan Malykh <ivan@roqua.nl>
Date:   Wed Oct 29 17:07:12 2014 +0100

    added feature scenarios for slider

commit cb7fe3679ff07e5f93b1924d5162267c17259f33
Author: Ivan Malykh <ivan@roqua.nl>
Date:   Wed Oct 29 16:56:41 2014 +0100

    pass the right value

commit 61563eb1bb40c63e4bcfbee1f59d451416da7714
Author: Ivan Malykh <ivan@roqua.nl>
Date:   Wed Oct 29 16:53:57 2014 +0100

    dont pass default_position if none

commit 3c00134f981b2e8aa370dccdb47ebb89b321cb1d
Author: Ivan Malykh <ivan@roqua.nl>
Date:   Wed Oct 29 13:49:33 2014 +0100

    use default position if available

commit 32043e05657e346692c67ff6fe63e0972945b5a2
Author: Ivan Malykh <ivan@roqua.nl>
Date:   Wed Oct 29 13:48:18 2014 +0100

    pass default_position with the slider data

commit 4490d884698ddd953c863a50e53b248822b6b8aa
Author: Ivan Malykh <ivan@roqua.nl>
Date:   Wed Oct 29 13:47:37 2014 +0100

    added DSL to override the default_position of the slider