F5-enhancement: multiline vertical down-copy

[vit_r]

F5 usual behavior is changed only when cursor is between

start and end marks (start_mark <= cursor && cursor <= end_mark)
of marked area.

Instead of copying selection inside itself
this patch offers one-click-down-copy of all lines
which have at least one highlighted char or '\n'.

New copy is marked and cursor is on the first char of lower line.

If nothing is marked then cursor line is doubled and marked.
In last case line with single '\n' is skipped.

vit_r

[vit_r]

F5-enhancement: multiline_vertical_down-copy