gnucash maint: Restructure early locale initialization

[Geert Janssens]

Updated	 via  https://github.com/Gnucash/gnucash/commit/6537135e (commit)
	from  https://github.com/Gnucash/gnucash/commit/eab06251 (commit)



commit 6537135e7072b39c90f065d04eec7b61810382c5
Author: Geert Janssens <geert at kobaltwit.be>
Date:   Thu Mar 18 14:41:40 2021 +0100

    Restructure early locale initialization
    
    - Move Macos specific bits to its own source file. As this is objective-c use .mm extension
    - Use common function signature for Macos and Windows init function and include via common header file



Summary of changes:
 gnucash/CMakeLists.txt                             |  12 +-
 gnucash/gnucash-core-app.cpp                       | 193 +-----------------
 gnucash/gnucash-locale-macos.mm                    | 222 +++++++++++++++++++++
 .../gnucash-locale-platform.h                      |  13 +-
 ...h-windows-locale.c => gnucash-locale-windows.c} |  56 +++---
 po/POTFILES.in                                     |   2 +-
 6 files changed, 269 insertions(+), 229 deletions(-)
 create mode 100644 gnucash/gnucash-locale-macos.mm
 copy libgnucash/backend/xml/gnc-entry-xml-v2.h => gnucash/gnucash-locale-platform.h (76%)
 rename gnucash/{gnucash-windows-locale.c => gnucash-locale-windows.c} (61%)