Set up Inkscape support as a meson subproject

[wpkelso]

This wouldn't get distributed in elementary OS itself, but rather be built every release and submitted to be included in Inkscape itself.
Keeping it here allows better control of the project, while still keeping the main icons theme fd.o specific.

Todo:

• Transfer Inkscape specific files from main project to relevant folders in subproject
• Make sure the build and release flow doesn't interfere with existing workflows

[wpkelso]

Closing as out of scope, opting instead to move maintenance of the theme upstream to Inkscape.