[clock app] Add city clock from toolbar

[Calum Pringle]

Currently, the only way to add a clock from another city is through edit. - see http://design.canonical.com/2013/03/app-patterns-applied-clock-key-journeys/ "clock: Adding a new city"

This is inconsistent with the journeys for adding an alarm and adding a timer. Add city should be available in the toolbar next to edit, and removed for now from the edit journey. Tapping on add city would jump to the list of locations with search at the top, and cancel button in the bottom left (rather than a back button).

[Ubuntu Phone Apps Jenkins Bot (ubuntu-phone-apps-jenkins-bot)]

Fix committed into lp:ubuntu-clock-app at revision None, scheduled for release in ubuntu-clock-app, milestone coreapps-13.10-month-2