#2018 Only a single selection is maintained when switching between buffers

[felix]

Steps:

1. Open multiple buffers in SciTE.
2. In one of them, make multiple selections (say, create a rectangular selection).
3. Switch to another buffer and back.
4. Notice only one selection was maintained in the buffer.

[Neil Hodgson]

If someone wants to work on this, the SelectionKeeper class in src/SciTEIO.cxx may be a good starting place.

[Neil Hodgson]

This was implemented with [074fb3].