lazy locale initialization

wx.Locale() can't be called without wx.App. So don't call it at
import. It makes PLCOpenEditor simpler.

use CustomIntCtrl instead of wx.SpinCtrl because it doesn't support
long integer needed for UDINT, (U)LINT types.

This commit complements commit 74a3f4d "fix LINT and ULINT ranges".

add custom TextCtrl allowed to enter long integer with bounds checking

It's needed because wx.SpinCtrl supports only 'int' type. For UDINT
and (U)LINT long type is necessary.

use pytest for testing