Fold remaining alt bindings into their main fields

Convert the remaining *Alt/*Alt[12] sibling fields (PrevItem/NextItem,
GotoTop/GotoBottom, PrevBlock/NextBlock, ScrollUpMain/ScrollDownMain,
OptionMenu, ConfirmInEditor, DiffingMenu) so the merge mechanism folds
their values into the corresponding main multi-key binding at config
load. The redundant alt-only Binding registrations across the various
controllers and the global keybindings file are gone: the merged main
field already carries every key, so the for-loop in SetKeybinding
registers them all.