[Enhancement] Cover Image Downloader: Save cover to disk/data folder

[ownedbycats]

Technically two enhancements, both for the context menu on a cover image:

a) Save cover to disk. Self-explanatory.

b) Save cover to data folder. Would be useful for people who save alternative covers. a) would mostly cover it, this is mostly for convenience.

Alternative: For a), perhaps the default could be the data folder if the current book. This may be annoying if it doesn't exist and the user plans to save elsewhere (resulting in empty data folders).

Thanks in advance.

[Kovid Goyal (kovid)]

Fixed in branch master. The fix will be in the next release. calibre is usually released every alternate Friday.