* refactor: s/tr_torrentFiles/tr_torrentFileProgress
Replace tr_torrentFiles() with a new function, tr_torrentFileProgress().
tr_torrentFiles() heap-allocated an array of progress structs. There is
nothing intrinsic in tr_torrent making batch computation more efficient,
so this PR replaces it with tr_torrentFileProgress(), a per-file variant
that doesn't use the heap.
* Rename GTK client .c files to .cc
* Fix GTK client build errors after C to C++ switch
* Remove `G_{BEGIN,END}_DECLS`
Co-authored-by: Charles Kerr <charles@charleskerr.com>