Better suggested signed name

When signing a "filename versioned" pdf, e.g. foo-1.2.3.pdf, the
current suggestion for signed name is weird. the "_signed"
gets misplaced in the middle of the version.

Also, compressed pdf's (pdf.gz) gets up with weird suggestions.

Fix both of these cases and document (by tests) the current behavior.

(cherry picked from commit c5909d0a)