vim: Consolidate gb_editor_vim_page_up and down functions

They use mostly shared code, so there's no reason for the repetition.

https://bugzilla.gnome.org/show_bug.cgi?id=738401
vim: Remove unneeded MAX macro in move_page up

It's never going to be less than 0.

https://bugzilla.gnome.org/show_bug.cgi?id=738401
vim: Add support for half page up/down (ctrl+u/d)

https://bugzilla.gnome.org/show_bug.cgi?id=738401