Try to make interpolate node movement a little nicer with curve points.

New key Alt+Ctl to allow toggling the current setting of interpolateCP
while the keys are held.

New modeline showing if Interpolate is active or not based on the setting
+ hotkey combination. That is, it shows what will happen right now.
Merge pull request #915 from monkeyiq/issues/893

Try to make interpolate node movement a little nicer with curve points.