Modernization + AZStd::function compare fix. (#3680)

* Modernization + small fix.

Modernize ( `bool`/`override`/other) code in AzCore, AzFramework, AzQtComponents, AzToolsFramework, etc.
Replaced a `bind` or two, use `using` in a few places as well.

Fix nullptr comparison of AZStd::function.

Signed-off-by: nemerle <96597+nemerle@users.noreply.github.com>

* Apply review-based changes

Signed-off-by: nemerle <96597+nemerle@users.noreply.github.com>

Code/Framework/AzToolsFramework/Tests/UndoStack.cpp

@@ -413,7 +413,7 @@ namespace UnitTest
             *m_completedFlag = false;
         }
 
-        ~UndoDestructorTest()
+        ~UndoDestructorTest() override
         {
             *m_completedFlag = true;
         }