Consistent use of "hovered" and style API conventions

We were still using "containsMouse" in a few places, and
we should consistently add "hovered" to all button types
for consistency. Also fixed the SplitView to follow the
new style conventions.

Change-Id: Iebc333c0cecc5bcde27e626b1a611c2f2edcf7dc
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>