Rework reactivity system & Introduce event system

[DavidVollmers]

• Rework and improve the reactivity system
  • Renamed the Value parameter of the ReactiveSection component to For
  • Introduced a base class ReactiveExpression for reactivity and a new class ReactiveReference to be used with reference types
  • Limited the ReactiveValue class to be only used with value types
• Introduce an event system which will be extended and used for later features

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (e58ace9) 43.44% compared to head (d5bdff7) 43.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   43.44%   43.90%   +0.46%     
==========================================
  Files          77       81       +4     
  Lines        2594     2619      +25     
==========================================
+ Hits         1127     1150      +23     
- Misses       1467     1469       +2     

Files	Coverage Δ
.../Ignis.Components.Reactivity/ReactiveExpression.cs	100.00% <100.00%> (ø)
...s/Ignis.Components.Reactivity/ReactiveReference.cs	100.00% <100.00%> (ø)
...ges/Ignis.Components.Reactivity/ReactiveSection.cs	92.59% <100.00%> (ø)
...kages/Ignis.Components.Reactivity/ReactiveValue.cs	100.00% <100.00%> (ø)
packages/Ignis.Components/HostContextBase.cs	100.00% <100.00%> (ø)
packages/Ignis.Components/IgnisComponentBase.cs	88.63% <100.00%> (+0.26%)	⬆️
...kages/Ignis.Components/IgnisComponentExtensions.cs	56.60% <ø> (ø)
....Components/Extensions/IgnisComponentExtensions.cs	0.00% <0.00%> (ø)

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