Added menu to move between levels

1) Added menu to move to previous/next levels or return to main menu.
2) Implemented dialog box to ask user whether app should continue
   retrying to generate a grid if it can't be generated after preset
   number of attempts.
3) Updated targetSdkVersion in AndroidManifest.xml to android-15.
4) Basic reordering of method/variable names to alphabetical order. For
   method names, first list constructor(s), then public methods (in
   alphabetical order), the private methods (also in alphabetical
   order). Not all files have been reordered.