Restructure ButtonBar

[neoruien]

• Refactor ButtonBar such that it extends UiPart instead of VBox
• Modify QuestionCard text

[codecov-io]

Codecov Report

Merging #195 into master will not change coverage.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master     #195   +/-   ##
=========================================
  Coverage     48.30%   48.30%           
  Complexity      783      783           
=========================================
  Files           155      155           
  Lines          3857     3857           
  Branches        477      476    -1     
=========================================
  Hits           1863     1863           
  Misses         1850     1850           
  Partials        144      144           

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/team/serenity/ui/MainWindow.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
src/main/java/team/serenity/ui/SideBar.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...java/team/serenity/ui/lessondata/QuestionCard.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)

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