GetPouVariables optimized with XSLTModelQuery

Added /plcopen/InstancesPathCollector.py, to move more model xslt querying code away from PLCController.py.

Fixed Exception dialog disapearing after a view second when exception occurs during app startup.
Problem was caused by splashscreen timeout, wx closing averything else if there is no more main frame.
Changes:
- no more timeout for splashscreen
- use wx.App OnInit method to give first operation to mainloop, object are then now created in mainloop
- main loop is then created _before_ showing splash screen
- no more wxyield or wx processevent tricks needed to display splash screen
- exception handler not blocking anymore on dialog (callafter)
- because of mainloop being there before everything, exit must be called explicitely if exception caught during startup -> exit parameter in handle_exception + try/except around startup calls

UNTESTED ON WINDOWS

Moved XSLTModelQuery class to own module in plcopen package.

Added a class to speedup all XSLT queries. WIP, now only for instances_path.xslt / GetInstanceList()

Stripped useless node creation in instances_path stylesheet.

This should really help GetInstanceList to perform better: resolver removed, no more deepcopy, no more useless xml serialization, no more node-set() calls.

YSLT auto indent trick is useless here, added yslt_noindent.yml2, regenerated XSLT files