Fix file function constness

[xezon]

Adds const to File member functions, but not yet the virtual functions.

[xezon]

Looks like your approval is yet non consequential :D

[feliwir]

Looks like your approval is yet non consequential :D

i thought approving twice can't hurt :) It asked me to add my review again:

[jonwil]

I have evidence that these were not const in the original. Is there a reason to deviate from that?

[xezon]

Yes. const correctness is important.

[xezon]

To elaborate on const correctness, here is article:
https://isocpp.org/wiki/faq/const-correctness

In a nutshell, anything that is meant to be immutable should be marked const, most importantly on

• immutable global variables
• immutable public struct members
• immutable private class members
• immutable member functions, that promise to not change state of callee
• immutable function arguments

const does not change the runtime.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (dbfc5bc) 2.53% compared to head (7e4ebc8) 2.53%.

Files	Patch %	Lines
src/game/common/system/file.h	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #786   +/-   ##
========================================
  Coverage     2.53%    2.53%           
========================================
  Files          949      949           
  Lines       110293   110293           
  Branches     18881    18881           
========================================
  Hits          2800     2800           
  Misses      107089   107089           
  Partials       404      404           

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