C++: drops IntSize in favor of Size<T> (#922)

Fixes #909

Since we declare the struct in C++, we don't need to declare it in
rust for cbindgen anymore, as long as we expose the Size2D type from
euclid to the cbindgen_private namespace

api/cpp/include/slint_size.h

@@ -0,0 +1,29 @@
+// Copyright © SixtyFPS GmbH <info@sixtyfps.io>
+// SPDX-License-Identifier: (GPL-3.0-only OR LicenseRef-SixtyFPS-commercial)
+
+#pragma once
+
+
+namespace slint {
+
+/// The Size structure is used to represent a two-dimensional size
+/// with width and height.
+template <typename T>
+struct Size {
+    /// The width of the size
+    T width;
+    /// The height of the size
+    T height;
+
+    /// Compares with \a other and returns true if they are equal; false otherwise.
+    bool operator==(const Size &other) const = default;
+};
+
+namespace cbindgen_private {
+    // The Size types are expanded to the Size2D<...> type from the euclid crate which
+    // is binary compatible with Size<T>
+    template <typename T> using Size2D = Size<T>;
+}
+
+
+}