Fix #500 Database update function should update description and category

- Include description and category in update_presentation function

Closes #502
Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>
Fix #508 Change Add New Talk button to a command button

- Move 'Add New Talk' button from TalkDetailsWidget to CommandButtons
- Connect new 'Add New Talk' button to confirm_add

Close #509
Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>
Remove unnecessary comment