#519 Update selected talk 3 seconds after last field change

- Add update_timer and set it to 3 seconds
- Add function start_update_timer, called on text/date/timeChanged for
  input fields
- Add function update_talk, called when update_timer expires
- Set SubmitPolicy for mapper to Manual
- Add function block_field_signals to prevent updating the DB when fields
  are autofilled
- Refactor add_talk and add function create_presentation to reduce duplicate
  code
- Add call to update_talk to closeEvent

Fix #519
Related proposal: 
https://docs.google.com/document/d/1VdPLpdIgteMfr1l7TPIZQ1M5XUKcy7ffVWu41t5zvjk/edit#