Capitalize description in DOAP file

To follow the convention.
Correct language codes of PO files

Also fuzzify invalid translations, otherwise git.gnome.org won't let me commit.
Fix gettext problems

GNOME's pre-commit hooks discovered a couple of gettext problems in
three translations.  This removes (by hand) the c-format specifiers
that were causing the problem as well as adds the no-c-format comments
in the code so the problem doesn't happen again.

Finally, msgfmt -c is now used to catch these errors in the future,
although the Makefile doesn't abort (like it should) when that
happens.  More work necessary for that.
Crash in video interpretable check: Closes #5071

The database and ThumbnailCache calls in check_is_interpretable
are not thread-safe although check_is_interpretable was being
called from a background thread.  Now check_is_interpretable returns
an object with a foreground_finish() function that does the thread-
unsafe work there.
Merge from shotwell-0.15 for 0.15.1
Build when "-Werror=format-security" used: Closes #719863
Add translator comments for strftime strings: Refs bgno#716533

The strftime strings for local-specific date/time formats are
cryptic, so hopefully these comments will help translators make the
right choice.
Merge po files, set up for intltool and l10n.gnome.org

This removes the custom .pot file setup we had before, making it easy to
translate Shotwell using the standard intltool workflow.

This also merges the two separate core/extras gettext domains into one,
to keep things simple.  I hope I've merged all the po files correctly,
but I wouldn't bet money I did it perfectly.

We also set up the help to be able to be translated on l10n.gnome.org.

Closes: bgo #718783
Updated Polish translation
Updated Spanish translation
Update Chinese simplified translation
Updated Galician translations
Updated Traditional Chinese translation(Hong Kong and Taiwan)
Updated Telugu Translations
Updated Greek translation
Updated Ukrainian translation
Tamil Translations Updated
updated kn.po
Updated Hungarian translation
Updated Hungarian translation