boulder: Add %gtk_update_icon_cache action macro

Most icon themes benefit from having their icon cache pre-generated at
build time.

The invocation for doing so follows a distinct pattern, hence this macro
could perhaps be gainfully used as a simpler substitution.

For convenience, the macro dependes on `binary(gtk-update-icon-cache)`.

Signed-off-by: Rune Morling <ermo@serpentos.com>

boulder/data/macros/actions/misc.yaml

@@ -1,5 +1,16 @@
 actions:
 
+    - gtk_update_icon_cache:
+        description: Pregenerate icon cache at build time
+        example: |
+            %gtk_update_icon_cache
+        command: |
+            for i in %(installroot)%(datadir)/icons/*/; do
+                gtk-update-icon-cache -f $i
+            done
+        dependencies:
+            binary(gtk-update-icon-cache)
+
     - install_bin:
         description: Install files to %(bindir)
         example: |