Add Maki icons

This patch makes use of the Maki point of interest icon set
from MapBox. The icons are BSD licensed.

https://bugzilla.gnome.org/show_bug.cgi?id=720250
place: make icon property read-only

The icon for a place is now derived from its type. Having the icon
property READWRITE does not make sense.

https://bugzilla.gnome.org/show_bug.cgi?id=720250