Making completion more interactive

[Tom]

Modern completion systems usually show completions
automatically. You just type and the completions are shown. This
is how it works in various applications, google, etc.

Currently, the builtin completion uses TAB for three purposes.

- showing the completion list for the current input
- completing the common part of the completions
- scrolling the completion list

The 2nd and 3rd could keep doing what it does, but in the first
case TAB could be eliminated, and completions could be shown
automatically as the user types either immediately or after a
slight delay when the user stops typing.

TAB completion could be kept for cases when completion is
costly (e.g. via TRAMP), but in cases where completion is done
from memory (e.g. M-x) completion candidates could be shown
automatically without having to press TAB to show them.