CppOverViewCombo: Fix scoll wheel behavior

The combobox only allows to switch between top level items via the
scroll wheel. Fix that by implementing a ::wheelEvent() handler.

Change-Id: I793a79322d9b00ebe30fc6e0452f8b383bb4a8a0
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
Move OverViewCombo to Utils::

Change-Id: I0724b6223ba9505effe151ce474f0f378671e60f
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
Utils::TreeViewComboBox: Add missing Q_OBJECT

Change-Id: Ie9a19af03172edebe67b6531864bffde799ab84c
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>