appview,spindle: group logs based on step
spindle logs now inform the beginning of a new step using a "control"
message, with logs categorized under a "data" message.
the appview in turn, is able to group logs by step:
- upon encountering a control message, it begins a new step (a
collapsible details tag)
- upon encontering a data message, it adds the log line into the last
encountered step
Signed-off-by: oppiliappan <me@oppi.li>