Created a resource file for plug-ins and a rule to link it to them.
The application icon file is smaller than the default one, there's no
point in doubling the size of a plug-in executable just for an icon.
If no problems turn up, this will close the bug.
This step moves the %.rc.o pattern rule to a small include file in the
build/windows/ directory. Michael Natterer suggested this approach in
order to minimize the changes done to each executable's Makefile.am.
Eventually all executables will have the manifest and version resources
linked. In this initial step, the current resources are moved from the
app/ directory to a global build/windows directory.
