convert FindInPouDialog from wx.Frame to wx.Dialog

In this case FindInPouDialog Windows isn't listed in windows manager
on Alt-Tab. And it's not shown on top of other applications. For
example, previously the dialog was shown even on top of web browser or
word processor.

Beremiz's user manual
=====================

Contents:

.. toctree::
   :maxdepth: 2

   install
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   edit
   build
   connectors
   debug