possibility: make it unnecessary to pass a function pointer into uiMainSteps()

split from #95

Actually looking at the GTK+ source code again, I wonder what would happen if I did this

``` diff
diff --git a/unix/main.c b/unix/main.c
index bfd0544..0269d86 100644
--- a/unix/main.c
+++ b/unix/main.c
@@ -41,7 +41,8 @@ int uiMainStep(int wait)
block = FALSE;
if (wait)
block = TRUE;
- return gtk_main_iteration_do(block) == FALSE;
+ gtk_main_iteration_do(block);
+ return 1;
}

// gtk_main_quit() may run immediately, or it may wait for other pending events; "it depends" (thanks mclasen in irc.gimp.net/#gtk+)
```

and ran `uiMainStep()` without `uiMainSteps()`. @gnarz? I could also try this myself I guess... but if this does work I could probably just manage the running flag myself like I do on OS X and avoid the need for a function pointer.