Close #504 Add About Widget to configuration tool

Add AboutWidget to the left hand tree on the config tool. This
widget is based off the previous AboutDialog. It contains information
about Freeseer, including copyright and version. There are also three
buttons, one links the user to the freeseer read the docs page
for help, the second other lets them report an issue on github and
the third button links to the contact page on read the docs.

The AboutDialog has also been modified. Essentially it was converted
to a widget (AboutWidget), which is used now for both AboutDialog and
the config tool.

Related to #411
Close #469 Add GStreamer state tests

Add 3 test cases to test GStreamer states: record, pause, and stop

test_current_state_is_record(): checks that the current state is RECORD
test_current_state_is_pause(): checks that the current state is PAUSED
test_current_state_is_not_stop(): sets state to null, then checks that
current state is not STOP

Related #437
Fix #523 Add New Talk button should open a popup for editing the new talk

The files removed in this commit were previously components of the Talk Editor
but are no longer being used.

- 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
- Refactor update_talk to use refresh_table function
- Set appearance of dialog (title, size, buttons, 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
- Remove unused files: AddTalkWidget.py, EditorWidget.py, UnlockTalkWidget.py