QmlDesigner.ComponentCore: Refactor and export AbstractActionGroup

The classes in modelnodecontextmenu_helper.h are used to create a
huge bunch of AbstractDesignerAction objects in a prototype based way
using function pointers.

This patch adds AbstractActionGroup which makes adding
sub menus easier, but does not use function pointers.

Change-Id: I5674493c16acf6fb86abd0f56c318ceff021e1b5
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.TabViewSupport: Add proper setter

Change-Id: Icc081ca3df73b565eae1aa062a86e3560f0b3793
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.ComponentCore: Making members private

Change-Id: I5674499c66acf6bb86abd0f56c318ceff021e1bb
Reviewed-by: Marco Bubke <marco.bubke@digia.com>