Image cache support project thumbnail

We need a thumbnail for recent projects.

Each project created by the latest wizard has a component call App.qml (content/App.qml).

We already generate a thumbnail for this component.

Whenever a project is opened an App.qml does exist we want to store the thumbnail in a seperate image cache. The key should be the project name.

In the welcome page we can check if the thumbnail exists (using the project name) and retrieve the thumbnail for the recent projects.

The thumbnail image cache can be used by a specific image provider in the welcome page.
The Studio Welcome Page already has a dependency on the QmlDesigner.