Adds Shortcut to move a point in a trajectory not neccessarily at the current frame

- Alt + holding left mouse-button enables to move the point under the cursor by dragging it with the mouse
- Point isn't moved anymore as soon as either the mouse button or alt are not pressed anymore
- Added documentation in "Key Bindings" Window for this and for !188 (Jumping to frame of trackpoint under cursor; Alt + dbl-Click)