Support GNOME's Damned Lies workflow

This should get Geary ready to be integrated into l10n.gnome.org,
GNOME's Damned Lies translation project.  The biggest change is that we
no longer track a .pot file, but we set it up so that translators can
generate their own using intltool-update --pot.

Closes: bgo #713827
Display plaintext parts along with inline image: Closes bgno#713846

Fix involved wider cleanup of Message processing code.  In particular,
RFC822Message.get_body() can allow or disallow the inline image
replacer to be the only part of the message depending on if get_body()
is in "fallback" mode or not.
Use ngexttext for "%d results": Closes bgno#721965
Updated Brazilian Portuguese Translation
Updated Hebrew translation
Updated Polish translation
Updated Czech translation
Updated Greek translation
Added dummy Makefile.am for compatibiluty with Damned Lies
Initial Spanish translation
Add missing ';' to the el.po
Updated Spanish translation
Updated French translation
Added missing line continuation char in help Makefile
Add semicolon to French translations of .desktop Keywords
Use ngettext() for plural forms: Closes bgno#722055
Merge branch 'master' into wip/721326-renormalize