Enhance Block Selection behavior

[PropGit]

Enhance editor so that any multi-line Alt-Block selection will be treated differently as follows:

• Instead of replacing the whole block with what is typed or pasted next, pressing any character or whitespace key simply inserts the character ahead of the block (in a single column that is as high as the block).
• If it's the Backspace key that is pressed, it deletes one column of characters from just before the left of the block.
• If it's the Ctrl+Backspace key combination that is pressed, it deletes all whitespace to the left of the block. This has the effect of moving the whole block (as a single item) left, as far as possible to bump up against the rightmost visible text left of the block.
• If it's the Delete key that is pressed, it deletes one column of characters from just after the right of the block.
• If it's the Ctrl+Delete key combination that is pressed, it deletes all whitespace to the right of the block. This has the effect of moving the visible text to the right of the block (as a single item) left as far as possible to bump up against the right edge of the selected block.
• CONSIDERATION If it's the Alt+Delete key combination that is pressed, it deletes all whitespace to the right of the block on a per-line basis. This has the effect of left-justifying (against the right edge of the selected block) any visible text that is to the right of the block.