Qt: only trigger call tips automatically on open parenthesis

call tips were being requested on every change to the document, causing performance problems
Handle some tricky Comm lifecycle issues

* If the comm was a primary comm, but there was an error opening it, _closed was False, which is wrong
* If Comm.close() was called, but an error happened, it still appeared to be open

This change makes the _closed attribute more conservative.  If _closed is False, the comm is definitely open for messages.  If _closed is True, we either didn't initialize correctly, or we tried to close at some point.
Unregister comm if there is an error in publishing the comm open message
add info-level log indicating that notebook is being executed
add `nbconvert --execute` flag

for enabling the ExecutePreprocessor
Merge pull request #6780 from minrk/nbc--execute

add `nbconvert --execute` flag
Merge pull request #6769 from minrk/qt-inspect-request

Qt: only trigger call tips automatically on open parenthesis
Merge pull request #6775 from jasongrout/comm-lifecycle

Handle some tricky Comm lifecycle issues
Fixed "data" to "text" change in message spec

This fix was accidentally omitted when the message spec recently changed.
Merge pull request #6788 from dsblank/patch-2

Fixed "data" to "text" change in message spec
Merge remote-tracking branch 'upstream/master'