GDK W32: Support switching input modules at runtime

This leverages the normal input module switching mechanism in GTK
by making it think that the gtk-im-module setting changed.
The backend returns gtk-im-module value as "ime" if W32
IME API says that an IME is in use. Otherwise it returns
and empty string - this still triggers an input module
loading code, which, not being able to load the desired module
(which is and empty string), falls back to looking at current
keyboard layout.

Paired with the code that signals gtk-im-module change on keyboard layout
switches, this is sufficient to make GTK capable of loading appropriate
input modules at runtime. At least, the kinds of modules that specify
languages for which they are loaded automatically by default, and the
IME module.

Loading other kinds of input modules might still work via specifying
the gtk-im-module setting in gtk ini file, but doing so will likely
make GTK incapable of loading the IME input module that is used
for Korean, Chinese and Japanese (and some other languages).

Until someone figures out a way to actually change gtk-im-module
setting on Windows at runtime with meaningful values, the behaviour
introduced by this commit seems like a sufficient workaround.

gdk/win32/gdkproperty-win32.c

@@ -389,6 +389,15 @@ _gdk_win32_screen_get_setting (GdkScreen   *screen,
             }
         }
     }
+  else if (strcmp ("gtk-im-module", name) == 0)
+    {
+      if (_gdk_input_locale_is_ime)
+        g_value_set_string (value, "ime");
+      else
+        g_value_set_string (value, "");
+
+      return TRUE;
+    }
 
   return FALSE;
 }