cellrendererpercent: subclass private progress implementation

We might get this subclassable in GTK soon, but until then just subclass
so we can move on and port more things.

src/libsysprof-ui/meson.build

@@ -24,6 +24,7 @@ libsysprof_ui_private_sources = [
   'sysprof-callgraph-page.c',
   'sysprof-cell-renderer-duration.c',
   'sysprof-cell-renderer-percent.c',
+  'sysprof-cell-renderer-progress.c',
   'sysprof-color-cycle.c',
   'sysprof-counters-aid.c',
   'sysprof-cpu-aid.c',