text: Title: Revise Application.cpu_usage() for atspi backend
  Body: The initial implementation of linux.Application.cpu_usage() provides general cpu usage as reported by `ps` linux utility. From `ps` man:
  > %cpu - cpu utilization of the process in
  > 	 "##.#" format.  Currently, it is
  > 	 the CPU time used divided by the
  > 	 time the process has been running
  > 	 (cputime/realtime ratio), expressed
  > 	 as a percentage.  It will not add
  > 	 up to 100% unless you are lucky.
  
  We might investigate for a better option to report CPU usage over the specified period.