Divide the whole of gtkwin.c into three parts.

Divide the whole of gtkwin.c into three parts.

This lays further groundwork for the OS X GTK3 port, which is going to
have to deal with multiple sessions sharing the same process. gtkwin.c
was a bit too monolithic for this, since it included some
process-global runtime state (timers, toplevel callbacks), some
process startup stuff (gtk_init, gtk_main, argv processing) and some
per-session-window stuff.

The per-session stuff remains in gtkwin.c, with the top-level function
now being new_session_window() taking a Conf. The new gtkmain.c
contains the outer skeleton of pt_main(), handling argv processing and
one-off startup stuff like setlocale; and the new gtkcomm.c contains
the pieces of PuTTY infrastructure like timers and uxsel that are
shared between multiple sessions rather than reinstantiated per
session, which have been rewritten to use global variables rather than
fields in 'inst' (since it's now clear to me that they'll have to
apply to all the insts in existence at once).

There are still some lurking assumptions of one-session-per-process,
e.g. the use of gtk_main_quit when a session finishes, and the fact
that the config box insists on running as a separate invocation of
gtk_main so that one session's preliminary config box can't coexist
with another session already active. But this should make it possible
to at least write an OS X app good enough to start testing with, even
if it doesn't get everything quite right yet.

This change is almost entirely rearranging existing code, so it
shouldn't be seriously destabilising. But two noticeable actual
changes have happened, both pleasantly simplifying:

Firstly, the global-variables rewrite of gtkcomm.c has allowed the
post_main edifice to become a great deal simpler. Most of its
complexity was about remembering what 'inst' it had to call back to,
and in fact the right answer is that it shouldn't be calling back to
one at all. So now the post_main() called by gtkdlg.c has become the
same function as the old inst_post_main() that actually did the work,
instead of the two having to be connected by a piece of ugly plumbing.

Secondly, a piece of code that's vanished completely in this
refactoring is the temporary blocking of SIGCHLD around most of the
session setup code. This turns out to have been introduced in 2002,
_before_ I switched to using the intra-process signal pipe strategy
for SIGCHLD handling in 2003. So I now expect that we should be robust
in any case against receiving SIGCHLD at an inconvenient moment, and
hence there's no need to block it.