Add align grid button to Viewport UI (#1311)

* first pass of adding grid snapping button

* update to request current grid size

* show/hide snapping option based on selection

* small tidy-up changes

* small updates following review feedback

* added some unit tests for snapping functionality and some small tidy-up/refactoring

* small refactor to ensure snap to grid ui only appears with snapping enabled

* add missing include to resolve build error

* fixes for build

* add & to make compiler happy

Code/Framework/AzCore/AzCore/std/math.h

@@ -26,6 +26,7 @@ namespace AZStd
     using std::exp2;
     using std::floor;
     using std::fmod;
+    using std::pow;
     using std::round;
     using std::sin;
     using std::sqrt;