Fix deprecation warnings

## Describe the bug
When building with the versions of these relevant packages:
- GTK 3.24.14
- Vte 0.58.3
- libgee 0.20.3

is see these deprecation warnings.

```
../src/Widgets/SourceView.vala:222.13-222.25: warning: Gtk.Widget.override_font has been deprecated since 3.16
../src/Widgets/SourceView.vala:222.13-222.25: warning: Gtk.Widget.override_font has been deprecated since 3.16
../plugins/brackets-completion/brackets-completion.vala:264.19-264.39: warning: Gee.Map.contains is deprecated. Use Map.has_key
../plugins/pastebin/pastebin_dialog.vala:325.31-325.45: warning: Gtk.Dialog.get_action_area has been deprecated since 3.12
../plugins/terminal/terminal.vala:255.17-255.26: warning: Gtk.Menu.popup has been deprecated since 3.22
../plugins/terminal/terminal.vala:262.13-262.31: warning: Vte.Terminal.spawn_sync has been deprecated since 0.48
../plugins/word-completion/completion-provider.vala:165.32-165.84: warning: Gtk.SourceCompletionItem.new has been deprecated since 3.24
```

## Platform Information
OS: NixOS

- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version