keyframes: Only consider a single clicks on keyframe curve
Only consider a single click (GDK_BUTTON_PRESS) to consider the keyframe curve clicked, instead of what has been done before, that is to also consider a double (GDK_2BUTTON_PRESS) or a triple (GDK_3BUTTON_PRESS) click.
It is logical to ignore double and triple click, because it would mean there was no motion involved.
Fixes #2566 (closed)