Attribute Support for Universal Variability Language (UVL)

[SundermannC]

Issue description

UVL comes with support for attributes that should allow storing the four types of feature attributed provided in FeatureIDE (long, double, String, boolean). However, currently, an integration of using feature attributes for UVL models is missing.
Feature models in UVL should be fully usable with the feature attributes view. Nevertheless, UVL should still be usable as regular feature model without attributes.

• Extend implementation to support reading and writing of attributes (cf. UVL Format).
  Hereby, a subclass of the regular UVL format should be created (i.e., ExtendedUVLFeatureModelFormat) in the attributes package.
• Implement distinction between regular and extended feature models
  The user should have the option to use UVL with and without feature attributes. Furthermore, the regular UVL format should be usable without the attributes package.

[h3ssto]

While there was some work on this issue in #1176, there are open questions. Therefore I will move this issue to 3.8.1.