The content of the horizontal Flickable element could remain at negative
positions after flicking and generally showed inconsistent behavior.
This was due to the redundant storage of the horizontal offset in
view.startX and flick.contentX. Explicitly assigning contentX from JS
code disables the automatic "rubber" effect which pulls the content back
into the allowed area.

Change-Id: I69e827854e2be3d8f6f4e2537e7105f80a8f2a89
Reviewed-by: Kai Koehne <kai.koehne@digia.com>