#427 Warn user about erasing new talk info

- Added function confirm_add, which is called when the "Add New Talk" button is clicked. If the Title, Presenter, Description, or Category fields contain text, a dialog box will request confirmation.

- Added helper functions are_fields_enabled and unsaved_details_exist for use in confirm_add. The prompt appears when these conditions are satisfied.

- Refactored clear_talk_details_widget to include category

- Included a docstring for unsaved_details_exist function

Fix issue #427 
Related issue #494 

Related to proposal
https://docs.google.com/document/d/1VdPLpdIgteMfr1l7TPIZQ1M5XUKcy7ffVWu41t5zvjk/edit#