opentrack 2.3 release candidate 15 patchlevel 1

• It's easier to contribute modules to opentrack, and to change its core functionality as the main areas have been documented. See: https://github.com/opentrack/opentrack/wiki/Hacking-opentrack
• Closing settings window by X in right top corner will cancel the changes.
• OSX builds now result in an app bundle, suitable for Mac users.
• FSX no longer stutters while in focus.
• On Linux, real camera names are used.
• "Open" button for camera settings on Linux now opens qv4l2.
• Opening a profile now switches to it, not the first profile in the new list.
• Calibration was removed for now because calibration files provided were invalid.
• Mac binaries for OSX 10.8+ are provided this time (tested by @alterscape)
• default settings for new users comprise of Accela filter and Freetrack protocol.
• Accela translation filtering was changed. Need testing if it made it worse. Previously it was too sluggish, now may be too sharp.
• patchlevel 1: unbreak PT tracking and calibration
• worked around crashes when start/stopping webcam trackers really fast