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 in LibraryConstants: 
    /**
     * The value for the mapsdir-directory. 
     */
    public final String DEFAULT_MAPSDIR = "maps"; //$NON-NLS-1$

and implemented in ResourcesManager and CustomMapsFolderPreference