build system: add additional automatic metadata

Add "gettext-domain", "extension-id" (containing the base part of the
UUID) and "gschemabase" (which, combined with "extension-id", gives
the GSettings schema) to metadata.json, autogenerated by the build
system.

extensions/workspace-indicator/metadata.json.in

@@ -1,5 +1,8 @@
 {
+    "extension-id": "@extension_id@",
     "uuid": "@uuid@",
+    "settings-schema": "@gschemaname@",
+    "gettext-domain": "@gettext_domain@",
     "name": "Workspace Indicator",
     "description": "Put an indicator on the panel signaling in which workspace you are, and give you the possibility of switching to another one",
     "shell-version": [ "@shell_current@" ],