qthelper.cpp is already quite voluminous. Move the recently
introduced localized versions of (v)snprintf() and put_format()
into their own translation unit.
Moreover, adopt C-style semantics for asprintf_loc(). This function
will be used to remove fixed-size buffers in core/plannernotes.c.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
