Place a pre-built copy of generated source files in src/built/, so that
users building SolveSpace from Git without the tools or setup necessary to
generate these can still complete the process.
Makefile.msvc: Use slashes consistently, and added rules to copy files from
src/built/ if needed
configure.ac: Check for presence of src/built/ at configure time
src/Makefile.am: Handle the generated *.table.h files together with
icon*.h; updated the source-generation rules so that the files are created
in builddir, not srcdir; added rules to copy files from src/built/ if
needed
The SolveSpace top-level directory was getting a bit cluttered, so
following the example of numerous other free-software projects, we move the
main application source into a subdirectory and adjust the build systems
accordingly.
Also, got rid of the obj/ directory in favor of creating it on the fly in
Makefile.msvc.
