Refactor effects page next/previous logic

Add is_next_effects_page() and is_previous_effects_page() to
CheeseWindow.
Fix an empty last page in the effects selector

If there were exactly the same number of effects available as were shown
in the effects selector, an empty last page was shown. Fix a condition
to test that there are more spaces for effects in the selector than
there are available effects. Fixes bug 720178.