UI and string changes in gnome-maps

Hi!

We are making a couple of changes to gnome-maps past UI freeze (after
approval from the release team).

The first is the public transit feature: https://bugzilla.gnome.org/sho
w_bug.cgi?id=755808

This has just landed in master, but the feature will not be immediately
  visible to end users, unless they set a debug environment variable,
until we enable the feature via the downloaded service file (which is
also used for the map tile addresses).

I have demoed this feature in my blog (and mirrored on p.g.o) https://m
l4711.blogspot.se/

There's some new strings:

For the search interface:

"Leave Now" - indicates searching for the next available transit trip
"Leave By" - indicates searching for transit trips departuring at
earliest a particular time
"Arrive By" - indicating searching for transit trips arriving no later
than a particular time

"Show" - Header in an options popover to select preferred modes of
transit

Strings for selecting preferred modes of transit

"Buses", "Trams", "Trains", "Subway", and "Ferries"

In the results list:

"Start" - Header indicating start of a journey, this string also has a
variant "Start at %s" where %s is known place name.
"Arrive" and "Arrive at %s" - for arriving in a journey.
"Show walking instructions" - tooltip on a button to reveal detailed
walking instructions on a leg of transit journey
"Show intermediate stops and information" - tooltip on a button to
reveal a list of intermediate stops passed by and further information
for a leg of a transit journey.

Error strings:
"No timetable data found for this route" - indicates that we no
timetable data available for the particular route.

The other feature (which hasn't landed in master yet) adds an
additional button to reverse a route: https://bugzilla.gnome.org/show_b
ug.cgi?id=777559
This feature doesn't introduce any additional strings.

//Marcus