#508 Change Add New Talk button to a command button

- Add 'Add New Talk' button to CommandButtons

- Remove 'Add New Talk' button from TalkDetailsWidget

- Connect new 'Add New Talk' button to clear_talk_details slot

Fixes issue #508
Related to issues #427, #494, #500

Related proposal:
https://docs.google.com/document/d/1VdPLpdIgteMfr1l7TPIZQ1M5XUKcy7ffVWu41t5zvjk/edit#