Make backends part of libkwin

Platform backends are provided as plugins. This is great for extensibility, but the disadvantages of this design outweigh the benefits.

The number of backends will be limited, it's safe to say that we will have to maintain three backends for many years to come - kms/drm, virtual, and wayland. The plugin system adds unnecessary complexity.

Startup logic is affected too. At the moment, platform backends provide the session object, which is awkward as it starts adding dependencies between backends. It will be nicer if the session is created depending on the loaded session type.

In some cases, wayland code needs to talk to the backend directly, e.g. for drm leasing, etc. With the plugin architecture it's hard to do that. Not impossible though, we can approach it as in Qt 6, but it's still harder than linking the code directly.

Of course, the main disadvantage of shipping backends in a lib is that you will need to patch kwin if you need a custom platform, however such cases will be rare.

Despite that disadvantage, I still think that it's a step in the right direction where the goal is to have multi-purpose backends and other reusable components of kwin.

The legacy X11 standalone platform is linked directly to kwin_x11 executable, while the remaining backends are linked to libkwin.