Insert SublimeGDB menu items into it's own menu labeled GDB in Context.sublime-menu

Having SublimeGDB menu entries displayed without a submenu makes the main Context Sublime menu cluttered.

I've made a simple change to the Context.sublime-menu to put all of SublimeGDB menu items into it's own submenu named GDB.

With this change, things will be cleaner and easier to tell which menu entries are from SublimeGDB.