Allow to interactively fetch / install examples

Allow users to select an example from the Welcome Scren that is only then downloaded.

Motivation

Currently the Welcome Screen only shows examples that are available on the local drive, usually installed through the Qt Online Installer, where documentation and examples for each Qt are installed by default.

Anyhow, this poses a problem for examples that have lots of large images etc: Adding them to every installation significantly increases the size of a default installation.

Options

The example could be an optional component of the online installer. The metadata of the example would be installed by default though. Selecting it in the Qt Creator welcome screen would let the Maintenance Tool install the example 'behind the scenes'.

Alternatively the example could also be directly fetched from a dedicated server, via either http or git. This is currently what's implemented in Qt Design Studio.