Fix entering characters at end of line in Replace mode

[ulyssa]

This resolves #206 by fixing several issues:

• Inserting into EditRope at the end of the line should continue inserting into the line, and not replace continue replacing bytes like the newline, or be stopped by the final character at the end of the buffer.
• Using backspace to restore characters should only pull text from the most recent history checkpoint once the cursor has moved back into the range of the replaced characters.
• Changelist entries need to be inserted after cursor adjustments are made, so that they do not get adjutsed.