pcbnew: Fix persistent multiple items selection box that stays after selecting a polygon/line (GAL).

Items that may be modified using EDIT_POINTs used to stay selected after pressing ESC (only EDIT_POINTs got away) - fixed.

Removed EDIT_CONSTRAINT::Update() - replaced with resetting constraints.
EDIT_CONSTRAINT is stored in EDIT_POIN using boost::shared_ptr instead of pointer.
Added EDIT_LINE::GetEnd() & GetOrigin(). Overridden ApplyConstraint() for EDIT_LINE.
Side EDIT_POINTS for zones are drawn as circles.

Switched order of EDIT_POINTs for DIMENSION, so in case when feature lines are of 0 length, then still can be stretched.

Dimensions have EDIT_POINTs.
Different handling of 45 degree mode in POINT_EDITOR.

Added POINT_EDITOR::isModified().

Minor code cleaning.