Enable automoc

Finally we can make use of this very convenient CMake feature and have
the moc stuff handled automagically. No need to list the headers anymore
for feeding them into the long-deprecated qt4_wrap_cpp macro.

Also, this should compile a bit faster and lead to slightly better optimized
code, because automoc will combine all the generated moc stuff for a target
into a single translation unit.

Note that we could get rid of various empty .cpp files (where all the code
is in the header) by just adding the header to the sources list; that way,
automoc will pick them up. There is no need to have a dummy .cpp file for
that.