report an error if ui file is not found
added missing option to man page
warn when signatures are valid but untrusted

taken from this commit:
https://git.gnome.org/browse/seahorse-nautilus/commit/?id=d440ec13198fb75f75996709bde32e4121ff8c9e
remove obsolete has_separator property from some dialogs
fixed README, removed the unneeded Ubuntu reference from changelog,
also removed useless debian/watch file
use GtkComboBoxText, remove check for ancient GTK+ versions
don't use deprecated g_atexit
prevent memleak and fix indentation a bit
NULL-initialize some vars to avoid logic flaws in the code below
Merge pull request #79 from monsta/even-more-fixes

[nemo-seahorse] fixes from upstream