Android kits are created only after restart (Or: Inform the user when a restart is necessary)

For example, after installing Android via Preferences -> Devices -> Set Up SDK, the user has to restart the application. This is e.g. necessary for the Android virtual device to show up in Preferences -> Devices -> Devices.

Creating a reusable "Qt Creator has to be restarted for the changes to take effect. Restart now?" [Yes] [Later] popup (or toast) that gets shown in such cases could be useful.