create capability for specifying custom maps folder

Mark,
Being that I'm working from a fork, the git commands are a bit different, but I did get it to rebase with spatialite_rasterlite2, so they should merge fine.   For prosperity, here's what I had to do:

git checkout mapsDirSettings
git fetch upstream
git rebase upstream/spatialite_rasterlite2
.. fixed strings.xml 
git add geopaparazzi.app/res/values/strings.xml
git rebase --continue
git branch mapsFolderName
git checkout mapsFolderName
git push origin mapsFolderName


Added further down
* where final value of `sdcard` is known:
   * when custom mapdir
      * check if custom-directory exists
         * if not: default maps-dir will be used (and created where needed)
         * assumption is: user may have deleted this directory
            * the application should **not**  create custom-directoes