ProductLicense.rb - display a checkbox instead radio buttons (bnc#878976)

make add-on license agreement similar to the base product EULA dialog
ProductLicense.rb - Display URL where to download the license (bnc#880685)
ProductLicense.rb - remove trailing spaces
changes

- 3.1.22
code review fixes

- use "eula_#{id}" as a widget ID
- use String#start_with?
added CheckBox shortcut
Merge pull request #57 from yast/eula_checkbox

ProductLicense.rb fixes