Added Fl_Preferences::file_access() and various flags that make it possible to limit or completely deny file access to the preferences system, either for the core library or for the application or both.
The Android example files in directory Android/... are not touched.
git-svn-id: file:///fltk/svn/fltk/branches/branch-1.4@12702 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
