Fix possible memory leak in CSS Classes of cells

[Marc-Spector]

I connected the profiler to the client.
As you can see from the graph, memory consumption increases dramatically during each minute, until the garbage collector starts its work. It looks like a memory leak.
After analyzing the code, I came to the conclusion that the styleClasses object is not a hash collection, which is why styleClasses contains many identical class names, which in turn triggers many monotonous actions (unexplained CPU spikes)

Before fix:

After fix:

[codecov]

Codecov Report

Merging #2524 (1dacaab) into develop (14d40ba) will increase coverage by 0.26%.
The diff coverage is 28.57%.

@@              Coverage Diff              @@
##             develop    #2524      +/-   ##
=============================================
+ Coverage      63.99%   64.25%   +0.26%     
- Complexity      4424     4483      +59     
=============================================
  Files            478      478              
  Lines          19301    19454     +153     
  Branches        1120     1144      +24     
=============================================
+ Hits           12351    12500     +149     
- Misses          6326     6331       +5     
+ Partials         624      623       -1     

Impacted Files	Coverage Δ
...va/com/faforever/client/fx/DualStringListCell.java	0.00% <0.00%> (ø)
...rc/main/java/com/faforever/client/fx/IconCell.java	61.53% <0.00%> (+2.71%)	⬆️
...ain/java/com/faforever/client/fx/NodeListCell.java	0.00% <0.00%> (ø)
...in/java/com/faforever/client/fx/NodeTableCell.java	0.00% <0.00%> (ø)
.../main/java/com/faforever/client/fx/StringCell.java	85.71% <ø> (-4.29%)	⬇️
...main/java/com/faforever/client/fx/DecimalCell.java	90.90% <100.00%> (-0.76%)	⬇️
...n/java/com/faforever/client/fx/StringListCell.java	47.61% <100.00%> (+2.16%)	⬆️
...ever/client/player/PlayerInfoWindowController.java	72.40% <0.00%> (-1.83%)	⬇️
...om/faforever/client/chat/ChannelTabController.java	81.71% <0.00%> (+0.38%)	⬆️
...ever/client/leaderboard/LeaderboardController.java	89.68% <0.00%> (+1.92%)	⬆️
... and 1 more

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