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

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          <xsd:attribute name="Command" type="xsd:string" use="optional" default="make -C %(buildpath)s all BEREMIZSRC=%(src)s BEREMIZCFLAGS=%(cflags)s MD5=%(md5)s USE_BEREMIZ=1 FROM_BEREMIZ=1"/>

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