Fix #523 Create New Talk popup

- Add NewTalkWidget class which has its own instance of TalkDetailsWidget
- Add instance of NewTalkWidget to TalkEditorApp and display it as a modal
  dialog when Add New Talk button clicked
- Add apply_changes function to repopulate the presentationModel and select
  the last updated talk in the table view with select_row function
- Add new talk to database when Add button clicked, close dialog and call
  apply_changes
- Refactor update_talk to use apply_changes, and prevent title-less talks
- Set dialog appearance (title, size, buttons, placeholder text, date format)
- Keep previously entered room, event, date and time in NewTalkWidget
- Remove unused functions from TalkEditorApp:
  add_talk, confirm_add, clear_talk_details_widget
- Remove unused files that were components of the old Talk Editor:
  AddTalkWidget.py, EditorWidget.py, UnlockTalkWidget.py