src: Mark symbols to be exported where needed

Generate export headers for the Quassel modules, and mark all
relevant classes and function to be exported so that shared libraries
can be linked against without globally exporting all symbols.
This is a hard requirement for Windows DLLs, and more efficient on
other platforms, too.

For now, this was done incrementally until everything linked properly.
In the future, we may consider explicitly defining the public
interfaces for each module, and trying to minimize the linker
interface e.g. by PIMPLing.