Extension helper and metadata are now different objects, so the
first needs be retrieved from ExtensionUtils.
Also, reduce the number of required arguments (by using the new
metadata keys) and make generic enough for usage by other extensions.
Includes documentation.
Common code for retrieving translations and GSettings schemas has
been factored out into lib/convenience.js, which is part of
every extension installation.
Since that code relies on renames done at zip file creation time,
extensions can no longer be installed with "make install". Instead,
one should create the zip file and install it with the tweak-tool.
There is also a bash script, local-install.sh, that will install
everything in zip-files.
Also, since the GSettingsSchemaSource code is not yet in a stable
GLib release, extensions using GSettings have seen their stable
shell version removed.
