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/CalcViewModel/CalcViewModel.vcxproj

@@ -448,4 +448,4 @@
     </PropertyGroup>
     <Error Condition="!Exists('..\..\packages\Microsoft.UI.Xaml.2.0.181018003.1\build\native\Microsoft.UI.Xaml.targets')" Text="$([System.String]::Format('$(ErrorText)', '..\..\packages\Microsoft.UI.Xaml.2.0.181018003.1\build\native\Microsoft.UI.Xaml.targets'))" />
   </Target>
-</Project>
+</Project>