Inkscape will read the "translationdomain" attribute from the
<inkscape-extension> root element in the .inx file.

It will then attempt to lookup a message catalog that matches
this domain, register it with gettext, and use it for translations.

Message catalogs may be located in either
- the .inx file's location
- the root of the "extensions" directory containing the .inx
- the system location Inkscape's own catalog is loaded from

To make this functionality available to script extensions, Inkscape
will set the environment variables
  INKEX_GETTEXT_DOMAIN
and
  INKEX_GETTEXT_DIRECTORY
so extension scripts can then use something like
  bindtextdomain(INKEX_GETTEXT_DOMAIN, INKEX_GETTEXT_DIRECTORY)
  textdomain(INKEX_GETTEXT_DOMAIN)
to enable the feature.

See also
  https://gitlab.com/inkscape/inkscape/issues/333
  https://gitlab.com/inkscape/extensions/issues/117

- Use identical logic for looking up <dependency>s and <command>s.
- Remove duplicate (but inconsistent and incomplete) logic from
  script.cpp that was used to search for the <command> again
- Remove <check> element from .inx format
  It seems unused (at the very least by core extensions) and
  redundant to <dependency> checking
- Deprecate the <command>-specific "reldir" attribute.
  Consistently use the functionally identical "location" attribute
  that was only used for <dependency>s before
- Introduce the new relative location value location="inx",
  which looks up <dependencies> and <command>s relative to the
  .inx file's location.

This is the "GUI" version of the executable that does not have a
console attached to itself and therefore doesn't flash a terminal
whenever it's called by an extension.

Functionality should be identical otherwise.

Fixed issues:
  - https://gitlab.com/inkscape/inkscape/issues/66

Makes searching through code for them easier.