effects: Modularize builtin effects build

Currently, cpp source files are included partially in effect sub-folders
and src/effects/CMakeLists.txt, which is really confusing and hard to
follow.

With this change, effect targets will be defined in their own subfolders.
This makes build files more straightforward.

Effect sub-targets are eventually merged into a monolithic
kwin4_effect_builtins target.

Another reason for modularizing builtin effects is that no two effects
can generate moc files with the same name atm even though they are in
different sub-folders. This can be potentially useful in the future, e.g.
making effects static plugins in order to decouple builtin effects from
libkwin (every effect subfolder would need a main.cpp or plugin.cpp file,
which will include the associated moc file).