Expose hovered to our controls and document it consistently

The current solution is messy and we only expose it partially to
some of our styles. I think we should simply expose this to all.

Change-Id: Ia546d5657ea416df99c9d9d92cc714b7f1c928fe
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Nico Vertriest <nico.vertriest@digia.com>