Fix #523 Add New Talk button should open a popup for editing the new talk

- 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 new talk to database when Add button clicked, close dialog and refresh
  table view
- Set appearance of dialog (title, size, button placement, placeholder text,
  date format)
- Add docstrings for show_new_talk_popup, refresh_table, add_talk and
  remove_placeholder_text functions and NewTalkWidget class
- Remove add_talk from TalkEditorApp