Adding a quick download button:

o Update row_doujin with a Button.

o New string for toast when already quick downloaded in
strings_download_manager_service.

o Added a new string for the intentQuickDownload.

o MainActivity is "notified" that quick dowload is done via the public
quickDownloadDone function.

o Using a boolean in the download function of the DoujinFragment class to
distinquish between normal downloads or downloads invoked by the quick download
button.