Add support for chapter updates

[manokara]

Chapter updates as in new versions of a same chapter - fixes from scanlators and etc. It seems MMN recognizes updates but they're just added to the top of the list. Here is an example. It wouldn't be a problem reorganizing the chapters (deleting updates from previous chapters) if it weren't for: 1) it takes quite some time; 2) the next update is going to add all chapters, from the first one, so I always end up deleting all chapters and downloading the list again.

When checking for updates, if the number of the chapter from the update list is less than the last chapter stored in the db, an "update available column" could be set in that chapter, an update label (like the "new" label), and a new button beside the trash icon, or a new entry in the menu to update the chapter, replacing the old images.

Summary:

• Check if new chapter is an update of and old one
• If it is, add an "update available" label to the already existing chapter entry in the DB
• An icon, or entry in the menu to update the chapter
• Replace old images with the new ones when asked to update the chapter.

I could even try to do that myself, but I don't have an android dev environment setup, and I'm a bit busy these days. Thanks for making this lightweight and awesome app :)