[FR] improve download

[ildar]

You made in-app downloading, this is great! And I'm sure it is possible to improve it:

1. change download target to getExternalFilesDir(). This requires no additional permissions yet brings many users (at least pre Android11) access to it without copying.
2. Open the downloaded files with e-book reader(s) through intent. Also a no cost operation.
3. loop through mirrors while downloading till success. Probably split "Download mirrors" button to "Download" and "Web download" for that.

Bonus notes (minor bugs):

1. Book name containing a small dash (-) is cut after that char for downloaded file name.
2. Navigation from Favorites to Search brings user to Downloads.