Updated, Curve Linedefs mode: distance/angle are now changed using bigger...

Updated, Curve Linedefs mode: distance/angle are now changed using bigger increments when being modified by mouse-dragging. You can hold Shift to enable increments by 1 unit.
Updated, Curve Linedefs mode: changing angle by mouse-dragging when "Fixed circular curve" option is enabled is now performed in a more predictable way.
Updated, Curve Linedefs mode: generated geometry is now merged with existing geometry.
Added, Curve Linedefs mode: a hint label is now displayed when modifying settings using mouse-dragging.
Added, Curve Linedefs mode: number of curve vertices can now be changed using LMB+RMB-drag.