Piano: Show a progress window when exporting WAV

This exposes that the export is pretty slow, but it's much nicer than
having the GUI lock up for 20s :^)

Userland/Applications/Piano/ExportProgressWidget.gml

@@ -0,0 +1,22 @@
+@GUI::Widget {
+    fill_with_background_color: true
+    layout: @GUI::VerticalBoxLayout {
+        margins: [4]
+    }
+
+    @GUI::Label {
+        name: "export_message"
+        text_alignment: "Center"
+        // FIXME: Change to dynamic width once that works.
+        min_width: 300
+        preferred_height: "fit"
+    }
+
+    @GUI::HorizontalProgressbar {
+        name: "progress_bar"
+        min: 0
+        max: 100
+        preferred_width: "grow"
+        min_height: 40
+    }
+}