Update simple_demo_qml coding style

[iche033]

🦟 Bug fix

A follow up to #373

Made a few coding style changes to be consistent with rest of Ignition code base as mentioned in this comment

2 space indentation
wrap line to 80 chars
use .hh and .cc file extensions
function separator style: //////////////////////////////////////////////////

see 2c7ac93 for the coding style changes before renaming the files.

I didn't touch ThreadedRenderer[.h|.cpp] as the code is mostly adapted from a Qt demo.

The only non-style change I made is adding these two lines below to set the env variable as mentioned in this comment

    // use single-threaded scene graph rendering
    qputenv("QSG_RENDER_LOOP", "basic");

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[codecov]

Codecov Report

Merging #404 (ca0a3bf) into ign-rendering5 (49e7a06) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head ca0a3bf differs from pull request most recent head ad17f2e. Consider uploading reports for the commit ad17f2e to get more accurate results

@@               Coverage Diff               @@
##           ign-rendering5     #404   +/-   ##
===============================================
  Coverage           57.77%   57.77%           
===============================================
  Files                 161      161           
  Lines               15947    15947           
===============================================
+ Hits                 9213     9214    +1     
+ Misses               6734     6733    -1     

Impacted Files	Coverage Δ
...e/ignition/rendering/base/BaseGaussianNoisePass.hh	100.00% <0.00%> (+3.33%)	⬆️

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