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

This change is necessary because the changes from #526 allow the Save Talk button in the Talk Editor to save modifications to existing talks. This means the process of creating a talk and the process of editing a talk use the same buttons and input fields. This may lead to some confusion because, once the input fields have been modified, there is no specific indication that an existing talk is being edited or that a new talk is being created.

Using this new popup will separate these two processes by keeping them in different windows and using different buttons and fields.

- 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
- Refactor create_presentation to accept a TalkDetailsWidget instance
- 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:
  confirm_add, clear_talk_details_widget
- Remove unused files that were components of the old Talk Editor:
  AddTalkWidget.py, EditorWidget.py, UnlockTalkWidget.py

This PR does NOT include unit tests.

This PR requires the updates from #526 to allow the save button to update existing talks
Related to issue #521
Relevant conversation on <a href="https://botbot.me/freenode/freeseer/msg/11948235/">IRC</a> 

Fixes #523