TaskTree: Introduce For element

Improve code readability by requiring iteratable Group
to be named For.

The For's c'tor requires an iterator element as a first arg.

The For's c'tor allows for passing exactly one iterator element.

It's not possible to place iterators inside Group element anymore.

Change-Id: I9dfe2c0da058abac161f66c4e336da2417c383f1
Reviewed-by: hjk <hjk@qt.io>

src/plugins/android/androiddevice.cpp

@@ -830,9 +830,9 @@ AndroidDeviceManagerInstance::AndroidDeviceManagerInstance(QObject *parent)
     // otherwise, Android Studio would give an error during parsing also. So this fix
     // aim to keep support for Qt Creator and Android Studio.
 
-    m_avdListRecipe = Group {
-        storage,
+    m_avdListRecipe = For {
         iterator,
+        storage,
         ProcessTask(onProcessSetup, onProcessDone)
     };
 }