Fix Ctrl+U and Ctrl+I being ambiguous in pages with frames, by only setting those shortcuts in the toplevel part.

I spent a few hours trying to make Ctrl+U show the source for the current frame while still seeing the shortcut associated with the "View Document Source" menuitem, but this was a complex shortcut reassignment mess.

Instead I added the [hidden, unfortunately] Ctrl+Shift+U for "View Frame Source".