Reset & re-init

Scheme agreed by James & Thorsten at FSWeekend 2012 to fix reset / re-init weirdness.

Motivation

• Re-init of all subsystems doesn't work, because many subsystems don't support it. (There's an open bug about this in the tracker). Instead we have 'reset', which does some special operations, but is different logic from normal startup. This leads to bugs when the reset behaviour differs to a fresh start.
• JSBSim has issues with resets since it doesn't read simulation state each frame - this is why you can't resume flying from a recording / replay for JSBSim aircraft.

Solution

• Get rid of reset / reinit entirely, and simply re-run the normal init sequence after tearing down almost everything. (What remains, is to be decided by testing - probably scenery, maybe nothing else at all).
• The requires reliable deleting + cleanup of subsystems - i.e fixing and fixing all the places using statics incorrectly. Some work towards this has already been done.
• The current property tree mirror needs to be replaced with some similar, but simpler logic, to preserve the user-archive properties, and re-overlay them after the property tree is rebuilt.
  • This is simpler than the current 'clone the entire property tree' logic which also causes bugs.
• Subsystems that remain alive must hence be checked that they unbind / re-bind correct, since we will get a new, clean property tree root.