HACKING: Recommend Glade 3.8.5, add link for fishman ctags fork

Also note that Glade callbacks don't need to go in `callbacks.c` anymore.
One thing we could suggest/do is to only put thin "wrapper" callbacks inside callbacks.c that don't really do anything but dispatch to a real function (mostly the case already). Since GtkBuilder callbacks are fairly limited (ex. user data must be a GtkBuilder object or the same for all callbacks) and it forces a specific function prototype. I'm thinking like this:

```c
// document.h
// fictional function
gboolean document_foobar(GeanyDocument*, GeanyProject*);
```

```c
// document.c
gboolean document_foobar(GeanyDocument *doc, GeanyProject *proj)
{
    // do whatever real stuff
}
````

````c
// callbacks.c
#include "document.h"

// fictional callback handler
static void on_document_widget_activated(GtkWidget *blah, gpointer unused)
{
    document_foobar(document_get_current(), app->project);
}
```

I don't think it's a big departure from the way stuff is currently. It would mostly just be documenting the pattern in `HACKING` (if it's not already). It also retains the advantages of separating GUI glue from real code and makes it dead easy to find the the callback handler if needed.