LanItems: replaced several globals using accessors.

Just to make things better testeable.
Testsuite for BuildLanOverview
DeviceStatus always returns unfrozen description string. bnc#906694
Testsuite extended for check on translation
Testsuite: improved formating
Simplified testsuite for BuildLanOverview
Cleanup: improved access to ifcfg configuration
Cleanup: droped some Builtins
Dropped call to NetworkInterfaces::device_num

- it is incompatible with new naming scheme
- it is completely useless at the place where it was used
Updated changelog
Merge pull request #272 from mchf/bnc906694-frozen-string

Do not modify frozen string