Fix #359 clean excess white space when adding talk

This removes excess white space before or after talk details
when adding a talk to the database.
Fix #427 warn user about erasing new talk info

- Add fieldsEnabled attribute to TalkDetailsWidget

- Add function confirm_add, called on addButton clicked
Remove Save button, change Add button to a command button, reconfigure Add button to call add_talk, change add_talk to insert presentations with empty titles and not disable input fields
Resolve conflicts