Fix #427 warn user about erasing new talk info

- Add fieldsEnabled attribute to TalkDetailsWidget

- Add function confirm_add, called on addButton clicked

- Remove fieldsEnabled attribute, add function are_fields_enabled and
unsaved_details_exist, refactor clear_talk_details_widget to
include category, prevent prompt when fields filled by talk selection

- Fix indentation

- Refactor confirm_add condition - removed unnecessary condition

- Fix indentation for confirm_add, are_fields_enabled, unsaved_details_exist
functions

- Add docstring for unsaved_details_exist function