Fire deleted notification on Buffer text replacement
Fix Cursor.forward over eof
GapBuffer#get_ptr always return zero terminated ptrs

Not zero-terminated at <size>, but at boundaries.
Fix potential nil ref when disabling cursor blink
Add some friendly error messages
Reset view position and selection on buffer switch
Remove unfounded assert from Buffer spec

Unclear why lines should stay the same after we moved the
gap with the Buffer knowing about it.
Ensure Cursor is in sync after deletions

I.e. move it to end if a deletion removed to much.