updates: Hide the last checked label if we haven't yet checked for updates

This prevents it from showing "Last checked: 1 January 1970".
Use G_DEFINE_TYPE_WITH_PRIVATE for GsApp
Use G_DEFINE_TYPE_WITH_PRIVATE for GsCategory
Use G_DEFINE_TYPE_WITH_PRIVATE for GsFolders
Use G_DEFINE_TYPE_WITH_PRIVATE for GsPluginLoader
Use G_DEFINE_TYPE_WITH_PRIVATE for GsProfile
Use G_DEFINE_TYPE_WITH_PRIVATE for GsScreenshot
Use G_DEFINE_TYPE_WITH_PRIVATE for GsShell
Use G_DEFINE_TYPE_WITH_PRIVATE for GsShellCategory
Use G_DEFINE_TYPE_WITH_PRIVATE for GsShellDetails
Use G_DEFINE_TYPE_WITH_PRIVATE for GsShellInstalled
Use G_DEFINE_TYPE_WITH_PRIVATE for GsShellSearch
Use G_DEFINE_TYPE_WITH_PRIVATE for GsShellUpdates
Show the repo name in the sources dialog

The repo-id is an implementation detail and isn't supposed to be shown to the
user.