KiCad leaves tombstone process

[Andrei-Pozolotin]

1. problem
   after plugin invocation, KiCad leaves tombstone process
   which must be killed manually by os task manager

2. workaround
   start scripting console before plugin invocation:

Pcbnew -> Tools -> Scripting Console

3. solution
   kicad should automatically provision global wx.App instance by default,
   similar to how it is done in kicad_pyshell
   https://git.launchpad.net/kicad/tree/pcbnew/python/kicad_pyshell/__init__.py#n92

See https://bugs.launchpad.net/kicad/+bug/1827742

[Andrei-Pozolotin]

further ideas by @qu1ck :

• use wx.PostEvent to resolve wx.App/wx.CallAfter problem
• See https://github.com/wxWidgets/wxPython/blob/master/src/_core_ex.py#L146
• See https://github.com/BrendanSimon/wxanythread/blob/master/wxAnyThread/__init__.py