EN: PsychoPy launcher improvements

[mdcutone]

This PR makes several enhancements to the PsychoPy launcher:

• The main PsychoPy app is now started in a sub-process with the correct environment for plugins to work without hacks to pkg_resources and site modules at runtime. Should be more compatible with py2app on Mac
• Packages can be installed/upgraded/removed prior to starting the main application so they aren't installed into a running environment. Likely possible to do in-place upgrades of PsychoPy in the future.
• PsychoPy is capable of self-restart after crashes or if initiated by the user: no more unrecoverable splash screen crashes
• Session lock file for determining if the app crashed in a previous session to allow for recovery tasks on the next startup

WIP or Planned features:

• Safe mode for recovering from startup crashes which disables plugins
• Hardware enumeration on startup in subprocess to avoid loading PTB and others with the GUI. Save hardware mapping to JSON in user preferences directory for populating dialog fields
• Command line options for clearing plugin cache, etc.