all of the selection needs to be redrawn so that is implmeneted in
Editor::InvalidateWholeSelection. Any extra selections should be discarded with
only the main remaining so that is Selection::DropAdditionalRanges.
Some default parameters led to less clarity so they no longer have default
values. Both Editor::MovePositionTo methods always returned 0 which was ignored
so they are now void. Some variables were made const.

to change in CancelModes.

When undoing many lines with markers (e.g. "changebar"
markers) LineMarkers::RemoveLine() is called for many
lines and as a result combining markers from the next line for all the
removed lines. This may cause the list contains many thousands of
elements and traversing it becomes expensive.

When lines are removed from the beginning to the end, it's better to prepend
the markers from the next line to the current line instead of appending them
because the current line "accumulates" all the markers from the following
lines and walking the whole list takes more and more time.

TargetWholeDocument.