Make the app cross-platform

[TolikPylypchuk]

.NET Core is cross-platform, so the service app can work on Linux. The settings UI should be implemented in Avalonia, and the ReactiveUI view models should be extracted into a separate project.

[TolikPylypchuk]

Supporting Linux seems more difficult than initially assumed. Closing this for now - will reopen if I ever decide to give it another try.

[TolikPylypchuk]

libuiohook can be used for hooking keyboard events instead of the Windows API. Alternatives for working with layouts should be researched (probably X11 can be used on Linux).