Add MIDI control points only in internal/pencil.
Revert "Create discrete lists for recorded MIDI controls."

This broke saving/restoring interpolation style.

This reverts commit 07a381a8f5572ae7550c60e5b1bd18c3dcdc7369.
Make recorded MIDI controls discrete, take 2.

Hopefully-desired behaviour is that controls created in the GUI are linear, so
clicking in stuff works like other automation, but controls that originated
from recording are set to discrete so Ardour plays back the input exactly,
instead of doing crazy things like linear interpolation of already high-rate
user input, hold pedals, and so on.

Hopefully that remains the desired behaviour, because we're basically screwed
for ever making any control discrete by default, since we only save the mode to
XML at all if it's not the default, which is currently linear.