Without this, the build fails with clang with
CFLAGS=-Werror=incompatible-function-pointer-types

```
../src/font-view-window.c:864:77: error: incompatible function pointer types passing 'void (FontViewWindow *)' (aka 'void (struct _FontViewWindow *)') to parameter of type 'GtkWidgetActionActivateFunc' (aka 'void (*)(struct _GtkWidget *, const char *, struct _GVariant *)') [-Wincompatible-function-pointer-types]
  gtk_widget_class_install_action (widget_class, "win.toggle-search", NULL, action_toggle_search_cb);
                                                                            ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/gtk-4.0/gtk/gtkwidget.h:956:87: note: passing argument to parameter 'activate' here
                                                         GtkWidgetActionActivateFunc  activate);
                                                                                      ^
```

Depends on https://gitlab.gnome.org/Infrastructure/static-web/-/merge_requests/23

When not logged in, the New Issue URL only redirects to an unhelpful
login page while the Issues page allows you to search for existing
issues and still offers a "New Issue" button.

The mnemonic also needs to be set in install_button_refresh_appeareance.

Closes https://gitlab.gnome.org/GNOME/gnome-font-viewer/-/issues/73

Part-of: <https://gitlab.gnome.org/GNOME/gnome-font-viewer/-/merge_requests/57>

Part-of: <https://gitlab.gnome.org/GNOME/gnome-font-viewer/-/merge_requests/58>

Part-of: <https://gitlab.gnome.org/GNOME/gnome-font-viewer/-/merge_requests/58>

Part-of: <https://gitlab.gnome.org/GNOME/gnome-font-viewer/-/merge_requests/59>

Remove ancient dconf sandbox holes, and add access to local fonts.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-font-viewer/-/merge_requests/67>

Part-of: <https://gitlab.gnome.org/GNOME/gnome-font-viewer/-/merge_requests/68>