Moved widgets definitions into widgets definition file
Removed unused 2nd stage proposals.

* second stage was removed from installer completelly
* net cards autoconfiguration is currently done other way
* selecting network service is dropped without replacement
  - SLE products are installed with one network service by default
* enabling/disabling IPv6
  - should be enabled by default, since IPv6 coverage is growing
  - if anyone wishes to disable it, he could do it via "Network
    Settings"
Adapted virtual proposals for first stage. bnc867451

It was originally done during second stage.
Cleanup: removed global variable Lan.virt_net_proposal
Updated changelog
Clenaup: dropped second stage specific code from LanClass#Write
Merge pull request #196 from mchf/bnc867451-network-autoconfig

Automatically setup virtual device when VM installed