Integrate a DSE backend selection mechanism in the DSE perspective.

* Exploration.alg:
  * Register the .alg plugin via an IStattup extension to avoid dependencies on this plugin.
  * Create a objective and a constraint evaluator that can handle DSML expressions.
  * Adjust the opt4j extensions.
  * Temporally introduce the MappingEncoding marker interface to support the DSML evaluators (--> allocation expressions).
  * Fix some minor bugs and warnings.
  * Move the porting utils to the alg plugin (previously in alg.ui).
  * Move the plot code to the alg plugin (previously in alg.ui) until the transition to the AF3 visualization framework is finished.
* Exploration.smt:
  * Create a backend implementation for the smt-based exploration.
  * Minor adjustments (I/O) to comply with the backend mechanism.
* Exploration:
  * Package renaming of the DSE backend mechanism.
  * Make the DSE backend mechanism independent of the eclipse extension mechanism.
  * Integrate the Exploration solution Classes in the EMF model.
  * Minor adjustments of the DSE backend API.
* Exploration.ui:
  * Add a combobox to select the DSE backend for the Deployment synthesis
  * Adjust the GUI backends to comply with the DSE backend interfaces.
refs 2939