Generate screenshots on the fly

Generate screenshots on the fly

This commit adds a new meson option -Dupdate_screenshots=true.
When it is enabled, and -Dgtk_doc=true is also used, then the
build will generate images to include in the API docs from
ui files in docs/reference/gtk/images.

Note: we still keep a copy of the images in git, in order to
allow building without a display connection. To update the
images in git, the generated images need to be copied back
from the builddir to the srcdir.