Multi-select is broken after s+t/s+b

The last non-shift selected key is not cleared out when executing s+t/s+b,
Also probably a problem with issue selection after . dialog operations.

We need to call selectionManager.setLastNonShiftSelectedKey(undefined) somewhere, but I'm not certain where. My initial idea was in selectionManager.clearSelectedIssueKeys(), but that had far-reaching consequences.