Fix Issue saving new assets in Asset Editor on Linux (#4537)

* Add helper function to apply a selected file filter from a file dialog to the result filename if needed
* Add platform traits to restrict the use of the helper function on platforms that need to apply it
* Fix building of file filters of multiple extensions for a file type

Signed-off-by: Steve Pham <spham@amazon.com>

Code/Framework/AzQtComponents/Platform/Windows/AzQtComponents/AzQtComponents_Traits_Platform.h

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+/*
+ * Copyright (c) Contributors to the Open 3D Engine Project.
+ * For complete copyright and license terms please see the LICENSE at the root of this distribution.
+ *
+ * SPDX-License-Identifier: Apache-2.0 OR MIT
+ *
+ */
+#pragma once
+
+#include <AzQtComponents/AzQtComponents_Traits_Windows.h>

Code/Framework/AzQtComponents/Platform/Windows/AzQtComponents/AzQtComponents_Traits_Windows.h

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+/*
+ * Copyright (c) Contributors to the Open 3D Engine Project.
+ * For complete copyright and license terms please see the LICENSE at the root of this distribution.
+ *
+ * SPDX-License-Identifier: Apache-2.0 OR MIT
+ *
+ */
+#pragma once
+
+#define AZ_TRAIT_AZQTCOMPONENTS_FILE_DIALOG_APPLY_MISSING_EXTENSION 0
+#define AZ_TRAIT_AZQTCOMPONENTS_FILE_DIALOG_FILTER_CASE_SENSITIVITY QRegularExpression::CaseInsensitiveOption