fixed missing prototype warning

From FR commit: fe3e6aa60ff370fb21475cf621fd86ad78c9303a
From: Paolo Bacchilega <paobac@src.gnome.org>
progress dialog: show the archive name in the main message..

and simplified the code a bit

Based on FR commit: 9e741444ddfc986a996bd20356fb0028d84eb347
From: Paolo Bacchilega <paobac@src.gnome.org>
engrampa doesn't delete directories w/ files within

consider '/path/to/dir' and '/path/to/dir/' the same path

Based on FR commit: 238b11888a6d88af70d056963257a7bf2cf4192c
From: Paolo Bacchilega <paobac@src.gnome.org>
Gnome bug: https://bugzilla.gnome.org/show_bug.cgi?id=632339

closes #69