Make the hardcoded ratings plugin threadsafe
Allow plugins to update an application icon
Never set the GsApp ID to the application ID

We want plugins to be able to set the ID from other metadata items if required.
Remove the horrific hack that makes search work without AppStream data
Set the GsApp ID from the DesktopDb if it is not set
Add an AppStream plugin

This allows offline applications to be shown in the software center.