Support selections with scrolling buffer

Selections now *mostly* work. They move as the buffer scrolls, copying
works as it should, and it looks like the different selection modes
behave properly as well.

The new Selection implementation uses buffer coordinates instead of
screen coordinates. This leads to doing a transform from mouse input to
update the selection, and back to screen coordinates when displaying the
selection. Scrolling the selection is fast because the grid is already
operating in buffer coordinates.

There are several bugs to address:

* A _partially_ visible selection will lead to a crash since the drawing
  routine converts selection coordinates to screen coordinates. The
  solution will be to clip the coordinates at draw time.
* A selection scrolling off the buffer in either direction leads to
  indexing out-of-bounds. The solution again is to clip, but this needs
  to be done within Selection::rotate by passing a max limit. It may
  also need a return type to indicate that the selection is no longer
  visible and should be discarded.
* A selection scrolling out of a logical scrolling region is not
  clipped. A temporary and robust workaround is to simply discard the
  selection in the case of scrolling in a region.

wip selections

fix issue with line selection

selection mostly working

need to support selection not being on the screen at draw time

Fix selection_to_string

Uncomment tests

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