gapplication: add GApplication:version property

This property is supposed to be used by authors of applications that use GAppliaction to output the version by --version flag or otherwise if a version is needed.

Closes #3198

Signed-off-by: Maxim Moskalets <Maxim.Moskalets@kaspersky.com>

gio/tests/gapplication-example-cmdline.c

@@ -34,6 +34,7 @@ main (int argc, char **argv)
                            G_APPLICATION_HANDLES_COMMAND_LINE);
   g_signal_connect (app, "command-line", G_CALLBACK (command_line), NULL);
   g_application_set_inactivity_timeout (app, 10000);
+  g_application_set_version (app, "2.3");
 
   status = g_application_run (app, argc, argv);