shortcuts: Enable Ctrl+PgUp/PgDn when multiple tabs

Users expect Ctrl+PgUp/PgDn to switch tabs, but some terminal
applications want to be able to use these keys themselves.  Make the
same compromise that gnome-terminal does, and let these keys switch
tabs _if_ there are multiple tabs in the current window to switch
between.

Fixes #118

.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/kgx-pages.c

@@ -819,6 +819,25 @@ kgx_pages_current_status (KgxPages *self)
 }
 
 
+/**
+ * kgx_pages_count:
+ * @self: the #KgxPages
+ *
+ * Returns: number of #KgxTab s in @self
+ */
+int
+kgx_pages_count (KgxPages *self)
+{
+  KgxPagesPrivate *priv;
+
+  g_return_val_if_fail (KGX_IS_PAGES (self), KGX_NONE);
+
+  priv = kgx_pages_get_instance_private (self);
+
+  return hdy_tab_view_get_n_pages (HDY_TAB_VIEW (priv->view));
+}
+
+
 /**
  * kgx_pages_get_children:
  * @self: the #KgxPages

src/kgx-pages.h

@@ -45,6 +45,7 @@ void        kgx_pages_add_page       (KgxPages   *self,
                                       KgxTab     *page);
 void        kgx_pages_remove_page    (KgxPages   *self,
                                       KgxTab     *page);
+int         kgx_pages_count          (KgxPages   *self);
 GPtrArray  *kgx_pages_get_children   (KgxPages   *self);
 void        kgx_pages_focus_page     (KgxPages   *self,
                                       KgxTab     *page);

src/kgx-window.c

@@ -260,10 +260,11 @@ key_press_event (GtkWidget   *widget,
    *
    * See https://gitlab.gnome.org/GNOME/libhandy/-/issues/422
    */
-  if ((keyval == GDK_KEY_Page_Up ||
-       keyval == GDK_KEY_KP_Page_Up ||
-       keyval == GDK_KEY_Page_Down ||
-       keyval == GDK_KEY_KP_Page_Down ||
+  if ((((keyval == GDK_KEY_Page_Up ||
+         keyval == GDK_KEY_KP_Page_Up ||
+         keyval == GDK_KEY_Page_Down ||
+         keyval == GDK_KEY_KP_Page_Down) &&
+        kgx_pages_count (KGX_PAGES (self->pages)) <= 1) ||
        keyval == GDK_KEY_Home ||
        keyval == GDK_KEY_KP_Home ||
        keyval == GDK_KEY_End ||