libgui directory Makefiles reorganized

[John W. Eaton]

I reorganized the Makefiles in the libgui directory.

Now we build libgui/liboctgui.la instead of libgui/src/liboctgui.la.
The new library is constructed from two convenience libraries,
libgui/qterminal/libqterminal.la and libgui/src/libgui-src.la.

You may want to clean out the old files in your build tree.

jwe