Rolling up the 0.4.0.0 development.

- Removed legacy references to BufferPosition and the integer-based system for MfGames.Commands.TextEditing LinePosition and CharacterPosition. +Changed
- Various corrections to get `Select All`, copying, and pasting working properly. +Changed
- Corrections to get the abstract text commands working with end of buffers and lines. +Changed
- Added extension methods to wrap the GetCharacterIndex and GetLineIndex in LineBuffer-specific calls. +Changed