Stop using GtkDialog (for most purposes) in GTK 3!

They've now deprecated gtk_dialog_get_action_area, because they really
want a dialog box's action area to be filled with nothing but buttons
controlled by GTK which end the dialog with a response code. But we're
accustomed to putting all sorts of other things in our action area -
non-buttons, buttons that don't end the dialog, and sub-widgets that
do layout - and so I think it's no longer sensible to be trying to
coerce our use cases into GtkDialog.

Hence, I'm introducing a set of wrapper functions which equivocate
between a GtkDialog for GTK1 and GTK2, and a GtkWindow with a vbox in
it for GTK3, and I'll lay out the action area by hand.

(Not everything has sensible layout and margins in the new GTK3 system
yet, but I can sort that out later.)

Because the new functions are needed by gtkask.c, which doesn't link
against gtkdlg.c or include putty.h, I've put them in a new source
file and header file pair gtkmisc.[ch] which is common to gtkask and
the main GTK edifice.

Recipe

@@ -207,7 +207,7 @@ GUITERM  = TERMINAL window windlg winctrls sizetip winucs winprint
 
 # Same thing on Unix.
 UXTERM   = TERMINAL uxcfg sercfg uxucs uxprint timing callback miscucs
-GTKTERM  = UXTERM gtkwin gtkcfg gtkdlg gtkfont gtkcols xkeysym
+GTKTERM  = UXTERM gtkwin gtkcfg gtkdlg gtkfont gtkcols gtkmisc xkeysym
 OSXTERM  = UXTERM osxwin osxdlg osxctrls
 
 # Non-SSH back ends (putty, puttytel, plink).
@@ -306,7 +306,7 @@ psftp    : [U] psftp uxsftp uxcons UXSSH BE_SSH SFTP wildcard UXMISC
 pageant  : [X] uxpgnt uxagentc pageant sshrsa sshpubk sshdes sshbn sshmd5
 	 + version tree234 misc sshaes sshsha sshdss sshsh256 sshsh512 sshecc
 	 + conf uxsignal nocproxy nogss be_none x11fwd ux_x11 uxcons gtkask
-	 + UXMISC
+	 + gtkmisc UXMISC
 
 PuTTY    : [MX] osxmain OSXTERM OSXMISC CHARSET U_BE_ALL NONSSH UXSSH
          + ux_x11 uxpty uxsignal testback putty.icns info.plist