use += instead of << for translations

_() result is frozen and << does not work
use "extension" term consistently (bnc#875199)
fixed plural form in a dialog title
Merge pull request #77 from yast/extension_term_bnc875199

use "extension" term consistently (bnc#875199)
Merge pull request #75 from yast/merging_translations

use += instead of << for translations
Merge remote-tracking branch 'origin/master' into addons_in_registered_system

Conflicts:
	package/yast2-registration.changes