Accelerator keys for zoom are not always fetched at menu level

When the key events for zooming are first fetched at canvas level,
they will not be propagated to the menu level on wxGTK.

This commit adds yet another workaround to the already existing long list.

src/document.h

@@ -972,6 +972,14 @@ struct Document {
                     }
                     return Action(
                         dc, shift ? (ctrl ? A_PREVFILE : A_PREV) : (ctrl ? A_NEXTFILE : A_NEXT));
+                case WXK_PAGEUP:
+                    if (ctrl) return Action(dc, A_ZOOMIN);
+                    sw->CursorScroll(0, -g_scrollratecursor); 
+                    return nullptr;
+                case WXK_PAGEDOWN:
+                    if (ctrl) return Action(dc, A_ZOOMOUT);
+                    sw->CursorScroll(0, g_scrollratecursor); 
+                    return nullptr;
                 #endif
             }
         } else if (uk >= ' ') {