allow registering extensions in installed system (bnc#876328)

(without re-registering the base product)

- removed BaseProduct - not needed anymore (the base product detection
  now works correctly)
- fixed connection initialization
remember registered extensions

- do not register them again
- disable the checkboxes (extensions cannot be "unregistered")
- display registered status in the main dialog
use += instead of << for translations

_() result is frozen and << does not work
added a helper method for accessing the registered addons
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
Merge pull request #74 from yast/addons_in_registered_system

Allow registering extensions in installed system (bnc#876328)
Merge remote-tracking branch 'origin/master' into registered_status

Conflicts:
	package/yast2-registration.changes
Merge pull request #76 from yast/registered_status

remember registered extensions
version update

- 3.1.50
use "modules" term as well (bnc#875199#c10)

- 3.1.51
Merge pull request #78 from yast/modules_term

use "modules" term as well (bnc#875199#c10)
allow adding media addons without registering the base system (bnc#870625)

- 3.1.52
Merge pull request #79 from yast/media_addons

allow adding media addons without registering the base system
fixed dependencies
Merge pull request #80 from yast/dependencies

fixed dependencies
fixed package dependency (it is 'yast2-add-on')

- 3.1.53