Up to now, we passed a "shiftPressed" flag to the individual
selection functions. To be more general replace by a struct
with "shift" and "ctrl" flags.
While doing this:
1) Move the struct into a new statsselection file for better
encapsulation.
2) Change shift to control in the scatter series, since individual
selection of items is usually done with control, not shift.
Shift usually means "select range".
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
