ComboBox: Make popup scrollable

We factor out part the menu item container logic from Menu into
ColumnMenuContent, which takes care of scrolling and mouse hovering,
and selection. This makes possible to extend the menu items layout.

The pop-over and pull-down look is specified by the menu style
component by overriding the ScrollView style. The popup's maximum
height is also specified by the menu style.

The gallery example can finally use a font families combo box.

Task-number: QTBUG-31568
Change-Id: I34a7278f476471c0eb51ef51dde3dd83e13002fe
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>