main: remove unnecessary code to handle last read page

The file handler class already takes care of it, and it does it better:
if the 'start from last page' confirmation popup is displayed, the
extraction thread will already have been started, so we don't waste time
waiting for the user choice.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@1077 ef8490bc-f317-4725-81ed-e95a9aca1bcf
pdf: another fix for optimal dpi detection

Simplify regular expression, so we don't miss some fill_image operations
because we can't parse floats like this: -9e-06

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@1078 ef8490bc-f317-4725-81ed-e95a9aca1bcf