[v 3.4.1] Toggle Guake visibility not working

# Toggle visibility using keyboard shortcut not working

```
Guake Terminal: 3.4.1
VTE: 0.42.5
VTE runtime: 0.42.5
Gtk: 3.18.9
```

## Steps to reproduce:

1. Open Guake
2. Toggle visibility to hide (`F12` or custom shortcut)
3. Toggle visibility to show (same key as above)

## Screenshots:

**Initial state**

**Active (visible) state**

**Hidden state, with toggle key pressed**