Add support for products of type 'mixed', which consist of both,
features and bundles, (like P2 supports them for a while), in the
Product-Editor and when launching Products from the editor.

The boolean-attribute 'fUseFeatures' is replaced by the enum-attribute
'type' whose allowed values are 'bundles', 'features' and 'mixed'.
All names are based on the model in P2.

To launch a mixed product from the IDE a feature based
launch-configuration is created and all product-plugins are simply added
as 'additional plugins'.

For the 'Contents' tab of the Product-Editor a new MixedSection is
introduced that allows modifying the Features and Plug-ins list of a
product simultaneously. The MixedSection does not support 'Add Required'
or 'Add working Set' actions, which is supported by the Plugin- or
FeatureSection support, to not overload the UI, keep the implementation
simpler. Furthermore adding requirements is not necessary because the
P2-Installer adds them anyway and therefore only makes the product
definition less flexible.
Displaying Features in alphabetically sorted order is also not supported
because it is more complicated to implement for mixed products and
likely a only rarely used feature.