fixed layout of stacklist in detailed flashcard view
modified getAllStack statement to order by stack AND front
created layout file for exporting stacks to a choosable filename and fileformat
added read/write permission
implemented Export UI
call of ExportActivity added
modified separator in fileString method
implemented import from txt file
modified some ids
created import layout
added ImportActivity
integrated the import layout, assigned to import activity, in the AndroidManifest and application logic