Now we use only new authorization keys for signing in. If we are
preparing to sign in and we see, that some of the authorization
keys were not generated in this launch, but were read from file, we
destroy all existing auth keys and generate a completely new set.
Some major platform-dependent file operations refactoring.
All methods like "open file", "open file with", "show in folder"
were moved to core/file_utilities module with platform-dependent
backends. All methods interacting with DesktopServices made async.
Now each platform-dependent module declares its interface in
platform/platform_module.h file and after that includes platform-
specific headers like platform/win/module_win.h with implementation.
Also removed the legacy WinRT platform-dependent implementations.
If we start logging in and we know, that some of the authorization
keys were read from the hard drive, not generated, we destroy all
the existing authorization keys and start generating new keys.
