Export JPEG and Canvas Size take the app's colour picker; a panel opens over a modal sheet (13.4a)

src/UI/ColorPickerSheet+Dialog.h

@@ -0,0 +1,22 @@
+#pragma once
+#include "Document/EditorSession.h"
+#include "UI/ColorPaletteControls.h"
+
+// Swift's DialogColorSwatch: a dialog's colour in the app's picker.
+class DialogColorSwatch : public SwatchButton {
+    Q_OBJECT
+public:
+    DialogColorSwatch(const QString &title, std::function<PaletteColor()> colour, std::function<void(const PaletteColor &)> change, EditorSession &session,
+                      QWidget *parent);
+    // Puts the picker away with the dialog, keeping its colour.
+    static void closePicker(EditorSession &session);
+
+protected:
+    // Swift's onDisappear: the picker goes with the swatch.
+    void hideEvent(QHideEvent *event) override;
+
+private:
+    EditorSession &m_session;
+    // Swift's onChange: the picker's colour as last seen.
+    std::optional<PaletteColor> m_seen;
+};