Remove Awkward Stretching of Config Widgets

When the config tool's window size is stretched, the components stretch
awakardly (ex. 4 lines will be evenly distributed over the entire
height of the window). This commit fixes this by adding a stretch
factor of 0 to the main layout.

Close #518
Related to #411
Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>
Fix #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