Fix apps menu not popping up when menu icons disabled

When disabling menu icons, it causes the menu to stop loading and reload. Unfortunately, this changes the behavior of gtk_widget_show(), which usually waits for the menu's initial load to complete before returning. Since the no-icons reload is not the initial load, it doesn't wait.

This fix creates the menu by first setting the show-menu-icons property before setting the GarconMenu, the latter of which is what triggers the initial menu load. This way, gtk_widget_show() will always wait for the menu to be populated, regardless of the icons setting.

Decided I'd do the same fix for the master branch as for 4.18. I can revisit doing something more complicated later if I really want to.

Closes #220 (closed).