This change adds dmenu's mixture of Emacs+vim-style Meta keybindings.
Also 'Page_Up' and 'Page_Down' were deprecated in upstream xkbcommon,
so replace them with the new 'Prior' and 'Next' names respectively.
There's no need to distinguish between vertical and horizontal mode
for the directional keys. By not doing so we match dmenu's behaviour
and also reduce code duplication.
