Handle PageUp and PageDown in the completion and popups

[MarSik]

This fixes three issues:

1. PageUp and PageDown during completion selection moved the
   viewport without cancelling the completion itself. It also
   left the last completion selected in the text.

   The change adds PageUp and PageDown handling to the existing
   movements (with the Ctrl-u and Ctrl-d aliases) to make
   the behavior of the editor and the menus consistent with
   each other.

2. Completion key actions were processed after the popup key
   actions. This prevented the PageUp / PageDown logic from
   being executed, because the popup swallowed them.

   The change reverses the logic, because the most active
   element should be handling the keys. Both code lens
   (space-k) and code completion now handle the page
   movements properly and consistently.

3. Popup Ctrl-u and Ctrl-d movements did not have the
   PageUp and PageDown aliases defined. This was confusing
   for new users as the editor itself recognizes those.