Merge libkwineffects into libkwin

libkwineffects was introduced when kwin used to be an executable. It
provided an api to implement effects and shielded from the technical
quirks in kwin.

Over the time, kwin internals had been split and abstractions were
refactored so they can be consumed in scripts or plugins. Besides that,
new ways to implement extensions have been introduced, which use
kwin's internal abstractions.

On the other hand, effects continue using libkwineffects specific apis.
This has a few issues: qtquick effects use both apis and it bites us,
duplicating same apis.

The best solution so far is to merge libkwineffects with libkwin, and
replace libkwineffects abstractions with libkwin abstractions, e.g.
EffectScreen -> Output, etc. This change takes care of adjusting libs.

Obviously, the main disadvantage of doing so is that binary effects
have to be recompiled every time new libkwin is released. But it's
already the case with libkwineffects too.