Tab subtitles and error message

[zawadz88]

This solves issue #160.
It adds an option to show subtitles in the tabbed stepper and to show an error message if verification failed.

[codecov-io]

Codecov Report

Merging #186 into develop will increase coverage by 2.82%.
The diff coverage is 70.1%.

@@              Coverage Diff              @@
##             develop     #186      +/-   ##
=============================================
+ Coverage      63.31%   66.14%   +2.82%     
- Complexity       183      198      +15     
=============================================
  Files             28       29       +1     
  Lines            946      963      +17     
  Branches          90       85       -5     
=============================================
+ Hits             599      637      +38     
+ Misses           292      281      -11     
+ Partials          55       45      -10

Impacted Files	Coverage Δ	Complexity Δ
.../stepper/internal/widget/ColorableProgressBar.java	69.23% <ø> (+6.18%)	9 <0> (ø)	⬇️
...one/stepper/internal/widget/DottedProgressBar.java	90.69% <ø> (+8.69%)	13 <0> (ø)	⬇️
.../java/com/stepstone/stepper/VerificationError.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...com/stepstone/stepper/viewmodel/StepViewModel.java	100% <100%> (ø)	9 <1> (+1)	⬆️
...epstone/stepper/internal/widget/TabsContainer.java	91.22% <100%> (+7.1%)	18 <9> (ø)	⬇️
...main/java/com/stepstone/stepper/StepperLayout.java	41.3% <22.22%> (+0.1%)	40 <1> (+1)	⬆️
...stepstone/stepper/internal/util/ObjectsCompat.java	33.33% <33.33%> (ø)	4 <4> (?)
...epstone/stepper/internal/type/TabsStepperType.java	66.66% <44.44%> (-10.26%)	4 <0> (-1)
...one/stepper/internal/type/AbstractStepperType.java	72.72% <50%> (ø)	4 <1> (ø)	⬇️
...com/stepstone/stepper/internal/widget/StepTab.java	67.08% <75.86%> (+14.9%)	26 <16> (+10)	⬆️
... and 2 more

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