py_ext: fix CSV Writer

fix POU logic :
- SAVE is a BOOL
- invocation of py_eval on rising edge of SAVE
- remove save python argument

fix python:
- use no encoding for file open (python2)
- re-use detected dialect if any
- use no "rt+" and truncate since no need to re-sniff dialect for output file
- return "OK" instead of "#SUCCESS", preventing POU logic to ACK result
- support creating new line if writing just after last line
- support appending data on short rows

fix example:
- use a HMI:Button to trigger CSV write instead of HMI:Input +1
- reload CSVs on on each new CSV opened in file browser
- add display of CSV write output

Widget syntax
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.. highlight:: text

HMI elements are SVG elements whose labels start with ``HMI:``::

    HMI:WidgetType[|freq][[:param]...][[@a=path[,min][,max]]...][#expression]

Widget Type
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Update Frequency
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Parameters
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Variable bindings
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Enable expression
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