World: show "World Weather" as a title when no city is selected

The header bar should never be empty, so add some filler text for
the default view.

https://bugzilla.gnome.org/show_bug.cgi?id=720322
City: use an icon instead of text for the back button

All other applications use an icon there, so be consistent.

https://bugzilla.gnome.org/show_bug.cgi?id=720321
AppMenu: remove application name from About

The new HIGs mandate About/Quit, in this order, without the application
name.
Consistently use the new website

Refer to the Apps space, instead of the design page.