Add Upgrade Notice for Themes

[afragen]

There is upgrade notice for plugins that displays on the core update page if $transient->upgrade_notice is set in the pre_set_site_transient_update_{plugins|themes} filter, but there is no corresponding upgrade notice used or available for themes.

I propose adding a similar usage for themes as currently exists for plugins.

Use case, when theme upgrades are pending a notice of what changes, etc. will be available from the main update page.

[afragen]

Any comments?