Integrate download an extraction and keep examples after download

Currently, the examples are download each time over and over.

Instead:

1) Download the examples once to QStandardPaths::Documents + '/QtDesignStudio'+ version.
This path should be the default but configurable in settings.
2) The download icon only appears if the example is already downloaded
3) When the example is opened and the first time it is also downloaded
– download and extration happens automatically. Afterwards the example is opened.
– To keep track of extraction progress we need the hardcoded number of files in the
ExampleModel
4) If an example is downloaded it can be redownloaded by clicking on the download/cloud icon.

The progress bar should have two stages. Download and Extraction. This requires a label.