ugtrain: move library files to src/lib

It is much better to see which files belong to the ugtrain logic
and which files belong to the multi-platform abstraction library
if the library files are put into a separate directory.
So move them to src/lib and sort includes alphabetically.
common: move common types to src/lib/types.h

We need multi-platform types for everything but we don't need the
other stuff in common.h for something else than ugtrain. So move
the common types to a new file src/lib/types.h and include it
everywhere.