Added hints and changed control point behavior, + minor fix to transferfunction

Added a hint item which displays the x and y values when dragging a point. It might be to large now, but can be changed when we find a more permanent location/size for the transfer function.

Also changed the graph to now restrict moving a point above/below each other. Apparently that is how they do it in paraview so all the reordering code has been removed 😞

Also fixed a issued with the applyTransferFunction() which would compute the wrong alpha values after I changed the graph range from 255 to 4029. Gradient on the graph now also reflect the changes!