Extend map scales down to support 1/32 zoom level.

Refactor all usage of 'GZ()' and *_mpp_to_scale() to use common definitions.
Including the addition of map_utils_mpp_to_zoom_level() and use it with WebToolCenter.
Prevent requests for downloading tiles at Zoom Level 19 and above for most map types

Allow MapQuest Zoom Level up to 19

TODO: This should be made a property of the map source and then use that value
 rather than this temporary hack.
Prevent the program grinding to a halt if trying to deal with thousands of tiles

This can happen when using a small fixed zoom level and viewing large areas.
Also prevents very large number of tile download requests.
Change map image interpolation to a more precise one.

Although this is slightly slower - it is not called often,
 not noticeable on modern systems and the result will get added the map memory cache anyway.

Reverse of commit SHA1:9fdb940f73cb138e942ddc38eb3f1854f6de70a2
Remove blank entry on the Tools->Webtools menu list.

Make ExtTools use GtkAction rather than tacking on GtkMenu items.