Ensure all documented examples are listed in Qt Creator

Currently examples do not show up in Qt Creator Welcome mode unless

• Example paths used in the module's .qdocconf and example docs are valid and match the install path
• Example documentation has at least one image (used as a thumbnail)

Especially the second one causes a lot of examples to not show up. To solve this, we need a mechanism to add a generic 'code sample' image to examples that we want to list.