Fix #494 Allow Add New Talk button to create an empty editable talk

- Remove Save button

- Change Add New Talk button to a command button

- Reconfigure Add New Talk button to call add_talk

- Change add_talk to clear input fields

- Change add_talk to insert presentations with empty titles

- Change add_talk to not disable input fields

- Disable input fields on talk deletion

- Update description and category in update_presentation

- Update presentation on text edited for line edit fields

- Add update_timer to put changes in effect

- Block signals to prevent continuous updating

- Update presentation on change for text edit and date/time edit fields

- Add function get_untitled_talks

- Add prompt warning user of untitled talks;
untitled talks are removed if response is 'Yes'

- Add function create_presentation to reduce duplicated code

- Add timer to handle close prompt in unittest