Do not abort if the AppStream XML is invalid

This allows the application to continue is there is one bad file.
Show a more correct warning message if the icon type is not supported
Add profiling functionality that can show a boot-chart log of startup
Do not load PackageKit in GsApplication
Show in the profile when the PK transaction was started
Do not search for icons in /usr/share/gnome-software/icons/hicolor
Set a cache age when using PackageKit

In this case, set a cache age of 'we don't care'.
Add some profiling to show AppStream loading

We do several things with the AppStream info at startup, and they all look like
they take a long time. In reality it's three threads all waiting for the XML to
be parsed, and the requests only take a very small time indeed.