add exponential falloff for reconnect

avoids constant reconnect attempts every 3 seconds forever

gives up after 6 tries (last timeout 64s)
add sticky `Connection lost` notification

when reconnect process gives up
add Reconnect to kernel menu
add events to ws_closed_error in kernel.js
wait for kernel_ready after ws_closed_error
s/Connection lost/Not Connected/
Merge pull request #6876 from minrk/falloff-reconnect

Add exponential falloff to kernel reconnect