Windows fixes

[vkbo]

This PR fixes three issues with running novelWriter on Windows, and one of the issues is general to all OSes.

• Generating the initial config file now works also on Windows. It previously failed since the config location required two directories to be created, but the code assumed only one was needed.
• On Windows, the Python open() command does not default to utf8, so all open statements now have encoding specified.
• Failing to open a file that exists, caused novelWriter to report an error, but still internally tagged the file buffer as open and ready. This is what it usually does for files that do not exist (i.e. new file), so the open statement in that case returns an empty string from the file load function. However, since the file load function did not distinguish between new file and loading error, it would assume this holds for error as well, causing the file to be overwritten with an empty string on the next auto-save, on save, or on file close. Now, the loading function returns None in case of an error, and the document wrapper class as well as the editor GUI class then clears its internal settings as if no file was open at all. This prevents the overwrite.

[codecov-io]

Codecov Report

Merging #81 into master will decrease coverage by 0.04%.
The diff coverage is 52.5%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
- Coverage    64.5%   64.45%   -0.05%     
==========================================
  Files          42       42              
  Lines        6319     6328       +9     
==========================================
+ Hits         4076     4079       +3     
- Misses       2243     2249       +6

Impacted Files	Coverage Δ
nw/convert/file/latex.py	36.66% <0%> (ø)	⬆️
nw/convert/file/concat.py	32.55% <0%> (ø)	⬆️
nw/gui/sessionlog.py	12.96% <0%> (ø)	⬆️
nw/convert/file/markdown.py	47.82% <0%> (ø)	⬆️
nw/convert/file/text.py	24.5% <0%> (ø)	⬆️
nw/convert/file/html.py	25% <0%> (ø)	⬆️
nw/tools/optlaststate.py	53.03% <0%> (ø)	⬆️
nw/project/project.py	82.44% <100%> (ø)	⬆️
nw/project/index.py	89.51% <100%> (ø)	⬆️
nw/project/document.py	87.32% <50%> (-1.25%)	⬇️
... and 4 more

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