require Vte 2.90 because these devs broke the API again

(they're really good at that)
Merge pull request #77 from monsta/gnome-devs-are-the-best-at-breaking-api

require Vte 2.90 (upstream API regression)
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
migration from GConf to GSettings
use GcrPrompt for prompting
Merge pull request #86 from monsta/seahorse-migration

[nemo-seahorse] upstream fixes - migration to GSettings and GcrPrompt
nemo-gtkhash: corrected one build-dep
nemo-image-converter: corrected one build-dep
nemo-preview: corrected build-deps
nemo-seahorse: bumped some build-deps
nemo-share: corrected one build-dep