Added handling for desktop upgrade

- bnc#874116
- defined in control file in software->upgrade->window_managers
Added new tests for new functionality
Changes and version
Changed according (most of) comments from review
And some more improvements according to PR
Merge pull request #19 from kobliha/update_options

Added handling for desktop upgrade
allow to specify target for AutoYaST upgrade via boot command-line (bnc#875031)
comment from review
Merge pull request #20 from jsrain/master

allow to specify target for AutoYaST upgrade via boot command-line (bnc#875031)