Reexport all of `gcore` in `gstd`, rename `image` node to `image_value`

[Firestar99]

Requires https://github.com/GraphiteEditor/Graphite/pull/2723

code changes, for bulk renames see https://github.com/GraphiteEditor/Graphite/pull/2729

This PR and https://github.com/GraphiteEditor/Graphite/pull/2729 makes editor to only depend on gstd and removes it's dependency on gcore. Instead, all of gcore is reexported in gstd. This allows me to rip apart gcore without editor being affected, as long as I continue to reexport all nodes in gstd. Saving us a lot of conflicts down the line.