GUI Tweaks: Set Qt5 default style to 'Fusion'

[vkbo]

Qt5 will try to set the application GUI style to something system appropriate, which doesn't interfere with the theme on at least Gnome, although it fails to render as intended on Unity, Windows 7/10, and MacOS. Setting the default style to "Fusion" resolves this at least on Windows. Need to test for others.

See Qt5 documentation: https://doc.qt.io/qt-5/qapplication.html#setStyle-1

This PR also adds the --style= flag to novelWriter, so that the default can be overridden by the user. It forwards this flag to QApplication's -style= flag. If the style isn't available, Qt5 handles this gracefully and just prints an error message and continues.

This solves issue #95, at least for Windows. Other OSes need testing.

[codecov-io]

Codecov Report

Merging #96 into master will increase coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
+ Coverage   62.77%   62.77%   +<.01%     
==========================================
  Files          42       42              
  Lines        6627     6630       +3     
==========================================
+ Hits         4160     4162       +2     
- Misses       2467     2468       +1

Impacted Files	Coverage Δ
nw/__init__.py	68.18% <50%> (-0.05%)	⬇️

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[vkbo]

Confirmed working on Ubuntu 16.04 / Unity as well.