trivial: Subsume category data when merging applications
Add a plugin helper for checking the distro-id
trivial: Do not show system apps in the installed view

I don't know why we didn't need this before, but with the plugin deps work we
started suddenly showing system applications. Filter them explicitly.
trivial: Add a tiny optimisation when calculating the plugin deps
Generates the 'Picks' on the overview automatically

This uses the AppStream data and any ratings data to choose applications based
on a primative metric.
Return all the featured applications in the hardcoded plugin

This lets us use the common date randomisation code used elsewhere.