[Graph] Replace the Add button by a EquationTextBlock (#826)

* init

* Replace the Add equation button by an EquationTextBox

* Move automatically the focus to the next textbox when users press Enter

* nit

* localize the placeholder

* rebase

* rebased and disable remove context menu when the equation is the last one

* replace storyboards by setters in visual states

src/CalcViewModel/GraphingCalculator/EquationViewModel.h

@@ -41,6 +41,7 @@ public
         OBSERVABLE_OBJECT();
         OBSERVABLE_PROPERTY_R(GraphControl::Equation ^, GraphEquation);
         OBSERVABLE_PROPERTY_RW(int, FunctionLabelIndex);
+        OBSERVABLE_PROPERTY_RW(bool, IsLastItemInList);
 
         property Platform::String ^ Expression
         {