Equation button updates: Enable/Disable on click, button content f1, f2, f3..., visibility icon on hover (#804)

* Added enable/disable line functionality

* Update EquationTextBox to change the opacity of functions have are not visible. Update the function label for the EquationTextBox to increment the label to show f1, f2, f3, etc

* rebase key-graph-features and fix issue where removing an equation box and adding a new one repopulates the previous equation

* Added visibility icon for the equation button hover

* updated EquationButton to be a toggle button to better handle the LineHidden state and other PR comment fixes.

* Updated EquationButton style to use a toggle button and to have placeholder icons for the show/hide states

* Updated equation button after pulling the refactor work into the branch. Fixed the Equation Button in KGF UI

* Fixed Pepe's bugs

* Uncomment temporary.pfx in calculator.vcxproj

src/GraphControl/Control/Grapher.h

@@ -173,7 +173,7 @@ public
         void OnEquationsChanged(Windows::UI::Xaml::DependencyPropertyChangedEventArgs ^ args);
         void OnEquationChanged();
         void OnEquationStyleChanged();
-
+        void OnEquationLineEnabledChanged();
         void UpdateGraph();
         void UpdateGraphOptions(Graphing::IGraphingOptions& options, const std::vector<Equation ^>& validEqs);
         std::vector<Equation ^> GetValidEquations();
@@ -210,6 +210,7 @@ public
         Windows::Foundation::EventRegistrationToken m_tokenEquationsChanged;
         Windows::Foundation::EventRegistrationToken m_tokenEquationStyleChanged;
         Windows::Foundation::EventRegistrationToken m_tokenEquationChanged;
+        Windows::Foundation::EventRegistrationToken m_tokenEquationLineEnabledChanged;
 
         static Windows::UI::Xaml::DependencyProperty ^ s_forceProportionalAxesTemplateProperty;