Fix memory leak due to missing free after gnc_ctime().

Also, remove the trailing newline from the returned time string, as this is rather unexpected here.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23183 57a11ea4-9604-0410-9ed3-97b8803252fd
Make gnc_g_date_time_new_from_unix_local() function known in header.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23184 57a11ea4-9604-0410-9ed3-97b8803252fd
After opening a file, display a statusbar message with the last modification date and time.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23185 57a11ea4-9604-0410-9ed3-97b8803252fd