Clear state when switching away, add blank state indicator

[krassowski]

Addresses #172 (comment):

• clear kernel usage state when switching away by listening to shell.activChanged when in JupyterLab
• implement informative blank state in place of incorrect "not available" blanket statments
• handle timeout (preserve old usage for reference but grey them out and show that timeout occurred)
• check delayed replies to prevent displaying outdated state for a previous notebook
• support consoles (as a bonus)

Demonstration of the new behaviour:

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