Sort unpurchased apps to top of "Up to date" section

Described by @chasinglogic [here](https://github.com/elementary/appcenter/issues/937#issuecomment-584836639):

>When I go to my installed apps I would like to see apps which I haven't purchased listed first. For instance I wasn't sure if this information was exposed via the installed list at all and so had to scroll through nearly the entire list to find LookBook which I wanted to purchase.
>
>It should be obvious and easy to find apps which I downloaded "on a trial basis" by paying $0 for. Especially for apps like LookBook which do not need to be updated often since if there is no update I will not get asked to purchase again.
>
>tl;dr I think unpurchased apps should simply be sorted to the top of the installed listing in AppCenter

It's separate and distinct from #937, which deals with the app info page.