Schematic: various improvements to dragging
Two things I think others should review:
- I changed SCH_LINE->GetPosition() to use the line's midpoint. Anything handling lines should really be specific about start point or end point if it wants it; I only found one place that used GetPosition() as a synonym for start point on lines.
- I renamed TEMP_SELECTED to SELECTED_BY_DRAG (DRAG_SELECTED was taken by the mouse logic).
Fixes: #10844 (closed)