Use Qt::endl with QTextStream

[ferdnyc]

Recent Qt releases complain about endl being used to terminate QTextStream lines, and recommend using Qt::endl instead. So, let's.

[codecov]

Codecov Report

Merging #555 into develop will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #555   +/-   ##
========================================
  Coverage    48.77%   48.77%           
========================================
  Files          129      129           
  Lines        10036    10036           
========================================
  Hits          4895     4895           
  Misses        5141     5141           

Impacted Files	Coverage Δ
src/CacheDisk.cpp	71.69% <100.00%> (ø)

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[ferdnyc]

Well, turns out Qt::endl wasn't a thing before Qt 5.14.0, so long story short, now we have a QtUtilities.h to go with our other compatibility headers. 😛

[ferdnyc]

This is incredibly minor, since it only impacts one source file (the only place we use QTextStream for output). We should perhaps be more consistent and use it in more places, but for now it's only CacheDisk. Point is, it passes Travis and should be fine. Merging.