[cmv4] Preserve multiple selections when saving new document or doing save as

[njx]

Noticed this bug randomly. @redmunds - trivial fix.

[njx]

BTW, I did a scrub through the code base for all calls to getSelection() and getCursorPos(). Other than this one and another case in Quick Open which I have a separate bugfix for (for #6965), all other calls to these APIs should be correct given the current state of the world:

• inline editors only deal with the primary selection
• some functions like find use the current cursor position to start searches
• code hints only use the primary cursor

So, overall, I think we've got all the places in the code that need to handle multiple selections, aside from this one.

[redmunds]

Merging.