*)  Add KIFACE_I::StartFlags() and IsSingle() so a KIFACE implementation can know
    if it is running under single_top.cpp or under a project manager.

*)  Test Kiface().IsSingle() when adding menus, some operations are not permitted
    when running under a project manager and the KIWAY_PLAYER is pegged to a
    specific project.

*)  Implemented KIWAY::KiFACE() so it loads *.kiface files.  They still have to be
    in the same directory as the main *.exe launcher, so this presents some difficulty
    when the binaries are not yet installed but rather the *.kiface files are still
    in their original build directories.  For today, I simply copied _pcbnew.kiface
    to build/kicad/.

*)  Add a test case to kicad/mainframe.cpp just to get an early peek at loading
    _pcbnew.kiface under the C++ project manager.  Got that working for one
    specific invocation just for proof of concept.  Surprise, it works.