Showing warning / question to users if they change the update method

- Only if changing from "patterns" to "update only installed rpms"
- BNC#873122
Changes and version
Default value (nil) for global variable doesn't need to be defined
Merge pull request #16 from kobliha/popup_for_switching_from_patterns

Warning user that changing update mode from "patterns" to "update only installed rpms" might lead into broken system.