Allow both Shift and Ctrl drag modifiers.

At least for bar line dragging, Shift-dragging can co-exist with Ctrl-dragging: allow detection of either, if both.
Fix merge conflict and modif. check for bar lines separate from other elements.
Merge pull request #1009 from mgavioli/Both_shift_and_ctrl_drag_modifiers

Allow both Shift and Ctrl drag modifiers.