When in a header and hitting Ctrl+PgDown, go to the end of the page if
there is no footer in it instead of staying in the header. The same case
for footers has been fixed too.

Removed the cursor positioning hackery in ToggleHeaderFooterEdit(): this
is now handled by the caller.

Show the Header/Footer edition when hitting Ctrl+PgUp and Ctrl+PgDown
and actually landing into a header or footer.