ПостTo clarify the level of trust a bit more, it's necessary to support only the legitimate sources instead of arbitrary sou…

To clarify the level of trust a bit more, it's necessary to support only the legitimate sources instead of arbitrary source and assign each a priority which are actually more challenging than the rests, because the definition of legitimate sources remains unclear to me, and without an exact definition, it's almost impossible to assign priorities as well. I initially came up with a priority list for the app stores, but I didn't explain why they were prioritized this way. Therefore, it is necessary to define leigitimate sources and factors that are needed to be taken into account to assign priorities. Again, this is a challenging problem, and for now, I'll rely on my own instinct for it. For the same reason, App Manager will not support any third-party updater extensions. The updater will rely on a signature permission to interact with the sources, and therefore, the extensions must be signed using the same key used to sign App Manager.