Fix memory leaks due to GMime bindings

Includes regen'd VAPI.
Conversation images don't show w/ Vala 0.23.1: Closes bgno#721000

Vala 0.23.1 introduced a change to how array .length property is
evaluated when transferring ownership to a method.  See
bgno#721001 for more information.
Scale inline images w/ regard to aspect ration: Closes bgno#713922

Required for images referenced by Content-ID.
Update docs, add install instructions

Closes: bgo #714717
Copy array manually to work around vala bug

Closes: bgo #720703
Add Fedora vala-tools dependency to INSTALL
Add separate trash/archive/delete buttons

Archive shows up as its own button for any account that supports
archiving (currently only Gmail).  Trash or delete shows up as one
button, depending on what the folder supports and whether you've got the
shift key held down.  Future work will extend supporting archive to
other account types and getting the trash special folder recognized in
more accounts.

Closes: bgo #714212
Update keyboard shortcuts in help

Closes: bgo #721792
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
Merge branch 'master' into wip/713846-plain-inline