ENH: Improve MeasureStartupTimes script

collect_startup_times_overall:

• Move existing code into "collect_startup_times_overall" function and
  save stats to a json file "StartupTimes.json" to facilitate analysis.

collect_startup_times_excluding_one_module:

• Added "collect_startup_times_excluding_one_module" starting Slicer N
  times excluding each one of the N modules one by one. Stats are
  then saved to "StartupTimesExcludingOneModule.json".

• List of module names (and their type) is also saved to "Modules.json"

• This allows to check if there are modules impacting the startup
  time more than the average.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25101 3bd1e089-480b-0410-8dfb-8563597acbee