Add an operator[](CharCount) to String and StringView
Make safe_ptr able to track callstacks

The code stays disabled, as the performance penalty is quite
high, but can be enabled to help debugging safe pointers.
Tweak client redraw logic, avoid highlight if only status line changed
Defer deletion of buffers to after the event loop

We can have SelectionList in flights on the buffer, so mark
it for deletion by moving it in a buffer trash, and effectively
delete it later, at a point where there should not be any further
access to it.