Freeze break request for Maps

[Jonas Danielsson <the sator gmail com>]

Hi,

I would like a freeze break for Maps. I would like to add the command
line option '--local' to use local tiles from a directory supplied.

The directory should have the standard...
   zoom-level/X/Y.png

structure.

With this people would be able to use available tools to download
tiles and will be able to use them in Maps using the --local switch.

It is in preperation for when we can allow downloading through Maps.
So it is a bit of a hidden feature. But I would like it in for testing
and for because it can be useful today.

There are a new string included in this. An error message when the we
cannot find the correct directory structure in the supplied path.

In lib/maps-file-tile-source.c:
"Failed to find tile structure in directory"

I have recoreded a cast of the feature that you can see here:
https://youtu.be/TWIwWYeE1u0

Where I open a diretory that contains Gothenbourg, zoom-levels 14,15,16 and 17.
The world is restricted to that bounding-box and those zoom-levels.

The code can be found on the branch: wip/jonasdn/local-tiles

Thanks!
Jonas