Fix337 tooltip synchronization

[dedeibel]

I tried to come up with a fix to #337

As suggested by Alex I tried to calculate all DataPoint information inside a read lock. One for each data set to keep it as small as possible.

[codecov]

Codecov Report

Merging #339 (94feb01) into master (fc81f1f) will increase coverage by 0.02%.
The diff coverage is 94.87%.

@@             Coverage Diff              @@
##             master     #339      +/-   ##
============================================
+ Coverage     51.74%   51.77%   +0.02%     
- Complexity     7300     7309       +9     
============================================
  Files           394      394              
  Lines         41197    41214      +17     
  Branches       6627     6625       -2     
============================================
+ Hits          21319    21339      +20     
  Misses        18364    18364              
+ Partials       1514     1511       -3     

Impacted Files	Coverage Δ	Complexity Δ
...in/java/de/gsi/chart/plugins/DataPointTooltip.java	72.22% <94.87%> (+4.33%)	31.00 <18.00> (+6.00)
...hart/plugins/measurements/DataSetMeasurements.java	75.77% <0.00%> (+0.34%)	153.00% <0.00%> (+2.00%)
...main/java/de/gsi/chart/ui/HiddenSidesPaneSkin.java	44.97% <0.00%> (+0.47%)	35.00% <0.00%> (+1.00%)

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[wirew0rm]

Looks good to me. I also like the readability improvements, thanks a lot. I'll merge this into master later today if there are no objections.