Preserve first line's leading whitespace on paste

This affects the behavior of copy/paste when `Auto Indent on Paste` is turned *off*, i.e. in whitespace-sensitive languages. I think it will make things more intuitive for people. Selecting groups of lines and pasting them at column zero now works as expected.

Note that the relative indentation of the copied text is still preserved on paste, as before.

This change removes one 'Public' method - `Selection::normalizeLines`. I thought of deprecating it, but it implemented a behavior that this PR removes, and no package calls it.

Closes #4272.