Served the app icon from the icon theme in the new element wizard

The qet theme now lists Nuri's app icon PNGs under the name
qelectrotech, and the wizard loads its logo by that name instead of
the 256 px file path. The files stay in ico/breeze-icons/, where the
packaging scripts install them from.

Part of #1269.

misc/make_icon_themes.py

@@ -314,6 +314,10 @@ BREEZE_PAGES = {
 PAGES = {name: f"breeze/{size}/{breeze}.svg" for name, (breeze, size) in BREEZE_PAGES.items()}
 PAGES["configure-shortcuts"] = "qet/pages/configure-shortcuts.svg"
 
+# The app icon's theme name. Its PNGs, one per size, stay in
+# ico/breeze-icons/<size>/apps/, where the packaging scripts install them from.
+APP_ICON = "qelectrotech"
+
 # Where the generator puts the 22 pixel Breeze art moved onto a 24 pixel
 # canvas (see on_24_canvas).
 BREEZE_24 = GENERATED / "breeze-24"
@@ -645,6 +649,13 @@ def main():
                     changed += write_if_changed(target, dark_image)
                     dark.append(f"themes/qet-dark/{size}/{name}.png")
 
+    # The app icon: Nuri's PNGs, used as they are. Colored art, so the
+    # dark theme inherits it.
+    for size in SIZES:
+        rel = f"breeze-icons/{size}/apps/{APP_ICON}.png"
+        if (ICO / rel).exists():
+            light.append((f"themes/qet/{size}/{APP_ICON}.png", rel))
+
     svgs = [((ICO / rel).name.replace("-symbolic", ""), rel) for rel in SVGS]
     svgs += [(f"{name}.svg", rel) for name, rel in PAGES.items()]
     for name, rel in svgs: