batch add: simplified the code removing useless variables

Based on FR commit: 2436a7e8fc36d4c020b566d1947b1571d5a49da2
From: Paolo Bacchilega <paobac@src.gnome.org>
Remove unused variable
set a fixed title for the progress dialog

Based on FR commit: fd497f4f9db806ce97f68d0366ae1f7fd37eccd1
From: Paolo Bacchilega <paobac@src.gnome.org>
add a left padding to the "other options"

Based on FR commit: 59f9149c4abb4f4fccbb7f22ae42d16ed45b4692
From: Paolo Bacchilega <paobac@src.gnome.org>
removed unused functions

Based on FR commit: 73fd3c56d567550be85574315d7a2beaec192970
From: Paolo Bacchilega <paobac@src.gnome.org>
FrCommand: set the filename before each call to fr_command_* functions

this change will allow the command to use a different filename from
the one pointed to by the local_copy attribute.

Based on FR commit: 5e48592fb49d1f5ce713fcd77d44acd664de4b6a
From: Paolo Bacchilega <paobac@src.gnome.org>
removed wrong call to fr_process_start

Based on FR commit: 1b1b0e719d39b0edd4fa6a91ade14ca69b9c0222
From: Paolo Bacchilega <paobac@src.gnome.org>
added a cancellable object to some GIO functions

Based on FR commit: a323c11554785a9861fff4ae919e91c076babd82
From: Paolo Bacchilega <paobac@src.gnome.org>
use local_copy instead of command->filename to check file existence

Based on FR commit: 77b2f368b8ab02267dc94bb70e7db16f37bbe252
From: Paolo Bacchilega <paobac@src.gnome.org>
renamed tmp_archive to old_archive

to improve code readability

Based on FR commit: 4f1f57d577388ec872f6d123cfaed03f0e812a82
From: Paolo Bacchilega <paobac@src.gnome.org>
removed useless call to fr_archive_stoppable

Based on FR commit: d5f88720970fdb16d81434607f312ea36a55674a
From: Paolo Bacchilega <paobac@src.gnome.org>
improved readability of code

Based on FR commit: c9ea52f8a3633a97ef64af1e18f9ff5b8e1474a9
From: Paolo Bacchilega <paobac@src.gnome.org>
allow the user to cancel addition and deletion of files

Based on FR commit: 815ad36da74abdc9e4948500af402e3f7d67e3ba
From: Paolo Bacchilega <paobac@src.gnome.org>
no need to force archive recompress

if the operation is canceled we can safely delete the archive as it
is just a copy of the original.

Based on FR commit: abfe5ef77bce70295e88bddc34972ac22cb270bc
From: Paolo Bacchilega <paobac@src.gnome.org>
progress dialog: show the number of remaining files to complete the operation

instead of the name of the file added/removed/extracted.

Based on FR commit: bf2b5d2352cfcd9d4345dda7976208672838dce1
From: Paolo Bacchilega <paobac@src.gnome.org>
make the Compress dialog larger

Based on FR commit: d2b320525916ef554a9f78d99e1687c1ccdadf9e
From: Paolo Bacchilega <paobac@src.gnome.org>
make the Compress dialog larger

Based on FR commit: d2b320525916ef554a9f78d99e1687c1ccdadf9e
From: Paolo Bacchilega <paobac@src.gnome.org>
use a boxed type to pass the error to the 'done' signal

Based on FR commit: 4b645f57c856928c508dd74c31e1bfb2132368e1
From: Paolo Bacchilega <paobac@src.gnome.org>
activate compiler warnings

Based on FR commit: 1ceb60e36079e1337aed053b62d327bc2e89fafc
From: Paolo Bacchilega <paobac@src.gnome.org>
fixed compiler warnings

Based on FR commit: 1becb98e4c8bc86feca13b54f991987c4eee3341
From: Paolo Bacchilega <paobac@src.gnome.org>