Fixed: in some cases lines with only back side were created after dragging map...

Fixed: in some cases lines with only back side were created after dragging map elements when using "Merge Dragger Vertices Only" drag mode.
Fixed: in some cases invalid sectors were created after dragging map elements when using "Merge Dragged Geometry" and "Replace with Dragged Geometry" drag modes, when at least one of dragged linedef was facing into an enclosed void area.
Fixed a crash after drawing lines over a sector with unclosed/invalid boundaries.
Fixed(?): probably fixed a crash when closing Script Editor with an unsaved file during program termination.