GraphControl: refactoring and optimizations

[rudyhuyn]

Description of the changes:

Full refactoring of Equation to use only normal properties instead of dependency properties

Refactor GraphingControl

• The EquationViewModel used for KeyGraphFeatures is now hosted by GraphingCalculatorViewModel instead of GraphControl

Refactor VariableViewModel to not use PropertyChanged as an internal event

• modify Min and Max even when Value is modified using the property
• remove OBSERVABLE_OBJECT_CALLBACK(OnPropertyChanged)
• replace the property Value and the method SetValue
• replace OBSERVABLE_PROPERTY_RW_ALWAYS_NOTIFY by OBSERVABLE_PROPERTY_RW
• extract VariableViewModel from GraphingCalculatorViewModel.cpp

Also

• use a Color instead of a SolidColorBrush for LineColor allowing us to remove the workaround using a blankBrush in EquationInputArea
  equationViewModel->LineColor = AvailableColors->GetAt(m_lastLineColorIndex);
• extract KeyGraphFeatures from Equation
• made many properties read-only
• rename EquationInputAreaControl.EquationVM as EquationInputAreaControl.SelectedEquation
• replace GraphControl::PropertyChangedEventHandler by the standard PropertyChangedEventHandler

How changes were validated:

• manually

[rudyhuyn]

INFO: This is a hack to allow us to use arguments with a comma in macros

[joseartrivera]

Looks good, left some minor comments, also looks like there are some merge conflicts that will likely require some manual changes. Thanks for working on this!

[rudyhuyn]

rebased!

[EriWong]

Buddy test passed and things look good.