Previously, more initialisation was performed inside DllMain but some actions
such as loading libraries are unsafe inside DllMain.
Avoid listbox UnregisterClass if no attempt to register which will occur if
no Scintilla windows were created.
std::call_once is used to ensure initialisation is performed at most once.

Read files as cp437 so all bytes are valid and windows-1252 invalid characters
do not fail.
When new headers found, save reasonable order of headers to "NewOrder.txt"
making it easier to update HeaderOrder.txt.
When a file is out-of-order, save list of headers used in the same order as
HeaderOrder.txt to "<path>.ordered".

scripts that require Python 3.6 have Python 2.x support removed.
Modules that may be called from Python 2 for PySide require Python 2.7.
Documented how to run scripts.

Remove unused headers from HeaderOrder.txt.