Add HIDE_FROM_SEARCH quirk
This allows plugins to control if apps are discoverable or not.
Discoverable here means that they get hidden from search results and category views, unless they are already installed. This makes it possible to launch and remove and update already installed apps, but hides them otherwise.
This is going to be used in the rpm-ostree plugin to hide package-based desktop apps on Fedora Silverblue and only show flatpak apps by default.