Can drag beziers.

Now, we need to recalculate the actual end point too, for natural curves.
Displacing arrow toward bezier. Very nice
Fixed bezier strength for new point drawing+dragging
Key to hide/show beziers
Flux colors for new reactions
Flux in reaction labels
New reaction at any point.

TODO Make a target symbol to show the starting point.
TODO Fix the undo/redo behavior
Potential fix for the modifiers keys 'sticking'
Target circle at new reaction point.
Fixed undo behavior for new_reaction_from_scratch
export SVG button in builder