Edit for mutable Qt mkspec can't be used

1. Go to "Tools" -> "Options..." -> "Build & Run" -> "Kits".
2. Select a Kit,.
3. Right click on its Qt mkspec edit and check "Mark as mutable" in the context menu.
4. Click "OK".
5. Open a project and configure it to use the kit from step 2.
6. Click on the target selector.
7. Try to type something into the "Qt mkspec" edit.
   You won't see what you type. Creator will write error messages to "General Messages" view.
8. Go to "Tools" -> "Options..." -> "Build & Run" -> "Kits".
9. Select the Kit from step 2.
   In its Qt mkspec edit, you can now see what you typed in step 7.

In step 7, what you type should be visible right away. If typing a mkspec is not allowed, it should not be possible to mark this as mutable in the Options dialog