Split keypad into its own widget

Testing Done:
Temporarily added a `GtkMenuButton` with a `GtkPopover` containing this widget to the media window, so I could check the buttons. Not sure that it totally works in the final window as I didn't start a real media session.

```
diff --git a/pidgin/gtkmedia.c b/pidgin/gtkmedia.c
--- a/pidgin/gtkmedia.c
+++ b/pidgin/gtkmedia.c
@@ -261,6 +261,14 @@
}

static void
+pidgin_media_dtmf_key_pressed_debug_cb(G_GNUC_UNUSED PidginKeypad *keypad,
+ gchar num,
+ G_GNUC_UNUSED gpointer data)
+{
+ g_message("DTMF key pressed! %c", num);
+}
+
+static void
pidgin_media_init (PidginMedia *media)
{
GtkWidget *vbox;
@@ -289,6 +297,27 @@
gtk_box_pack_start(GTK_BOX(vbox), media->priv->display, TRUE, TRUE, 6);
gtk_widget_show(vbox);

+ {
+ GtkWidget *button = NULL;
+ GtkWidget *keypad = NULL;
+ GtkWidget *popover = NULL;
+
+ button = gtk_menu_button_new();
+ popover = gtk_popover_new(button);
+ gtk_menu_button_set_popover(GTK_MENU_BUTTON(button), popover);
+
+ keypad = pidgin_keypad_new();
+ pidgin_keypad_set_key_capture_widget(PIDGIN_KEYPAD(keypad),
+ GTK_WIDGET(media));
+ g_signal_connect(keypad, "pressed",
+ G_CALLBACK(pidgin_media_dtmf_key_pressed_debug_cb),
+ NULL);
+ gtk_container_add(GTK_CONTAINER(popover), keypad);
+
+ gtk_box_pack_start(GTK_BOX(vbox), button, FALSE, FALSE, 0);
+ gtk_widget_show(button);
+ }
+
g_signal_connect(G_OBJECT(media), "delete-event",
G_CALLBACK(pidgin_media_delete_event_cb), media);

```

Reviewed at https://reviews.imfreedom.org/r/1565/