Modem: Deselect country when "Custom Providers" list is used.

Before the fix a country remained selected when working with "Custom
Providers" list. It would be a bit confusing bcs the list contains all
customized provider configurations regardless of country of origin.
Adopted for new version of yast2.rpm
Updated changelog.
Replaced "empty string checks" using function provided by yast2.rpm
Merge remote-tracking branch 'upstream/master' into bnc809053-modem

Conflicts:
	package/yast2-network.changes
Merge pull request #100 from mchf/bnc809053-modem

Adopted to new yast2