gs-star-widget: Split out rating refresh from other refresh

It’s quite expensive to destroy and recreate all the child widgets of a
`GsStarRating` every time its rating is changed. In particular, doing so
means re-loading the `starred-symbolic` image a lot.

Avoid that by separating refreshing the `rating` property from
refreshing everything else.

This avoids around 115KB of object allocations through refreshes.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
2 jobs for star-performance in 2 minutes and 54 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #747769
debian-stable-x86_64

00:01:22

passed #747768
fedora-x86_64

00:02:54