Scripting: Allow referencing built-in icons from Tool.icon

For example ":images/22/stock-tool-bucket-fill.png" to refer to the
Bucket Fill tool icon.

Note that these icons may change at will, there's no compatibility
guarantee. There is also no icon reference, these icons can currently be
found in the repository at 'src/tiled/resources/images'.

src/tiled/scriptedtool.cpp

@@ -260,7 +260,7 @@ void ScriptedTool::setIconFileName(const QString &fileName)
     QString iconFile = fileName;
 
     const QString ext = QStringLiteral("ext:");
-    if (!iconFile.startsWith(ext))
+    if (!iconFile.startsWith(ext) && !iconFile.startsWith(QLatin1String(":")))
         iconFile.prepend(ext);
 
     setIcon(QIcon { iconFile });