...and hook it up.
I opened MainMenu.xib in Xcode, added a new "Submenu Menu Item"
from the Library (cmd-shift-l), added a User Defined
"toggleShowClippingPaths:" action on First Responder and connected
the menu item's action to that action.
(I first tried duplicating the existing Window menu and editing that,
but the Window menu is marked as `systemMenu="window"` in the xib and
I couldn't find a way to undo that in Xcode. So the Debug menu first
acted as a second Window menu.)
I made "Debug" a toplevel menu to make it consistent with Ladybird.app
for now, but I'll probably make it a submenu of "View" in the future.
When the outline has focus, arrow keys navigate the outline instead
of changing the current page.
Add opt-up and opt-down as a way to move by one page even when the
outline has focus. (This matches Preview.app.)
xib change: Added two menu Previous Page with key equivalent opt-up
and Next Page with key equivalent opt-down to Go menu and bound them to
goToPreviousPage: and goToNextPage: on First Responder.
When the outline has focus, the responder chain is outline ->
window, so also add the actions on the window controller, and
let that forward to the PDF view.
