Show warning when installing apps that are not free

[yoshiask]

Currently, Fluent Store will happily download and install apps from the Microsoft Store that are paid, such as Affinity Designer. Note that does not enable piracy: Windows will prevent the app from running if the signed in user does not have a valid license for it. This can lead to a poor user experience, if they download and install an app only to find that it doesn't work. There should be a warning of some sort on paid apps to explain this. The real trouble will be implementing this while keeping the abstraction between the UI and package sources.