Implementation and tests for ordering of element attributes

[PAX523]

This behavior is optional.

[codecov-io]

Codecov Report

Merging #69 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #69      +/-   ##
============================================
+ Coverage     39.93%   39.99%   +0.05%     
- Complexity     1493     1499       +6     
============================================
  Files           145      145              
  Lines          9490     9497       +7     
  Branches       1192     1193       +1     
============================================
+ Hits           3790     3798       +8     
+ Misses         5320     5319       -1     
  Partials        380      380

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/org/dom4j/io/OutputFormat.java	63.63% <100%> (+1.56%)	32 <4> (+4)	⬆️
src/main/java/org/dom4j/io/XMLWriter.java	60.92% <100%> (+0.1%)	155 <8> (+1)	⬆️
...rc/main/java/org/dom4j/tree/ContentListFacade.java	29.78% <0%> (+2.12%)	8% <0%> (+1%)	⬆️

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[long76]

@FilipJirsak please merge this, thanks!