QML Designer thumbnails

Hi Diana, we are missing QML Designer thumbnails for the new controls. See the attached screenshot.

We need two different thumbnails in low-res and high-res versions (@1x and @2x) for each control. The larger thumbnail, in the "QML Types" list, is 24px. The smaller thumbnail in "Navigator" is 16px.

Notice that we don't have QML Designer support for some of the controls yet, so the screenshot below is missing some. The full list of controls is available at http://doc-snapshots.qt.io/qt5-5.6/qt-labs-controls-qmlmodule.html. It would be nice to have thumbnails for all so that we have them ready when we add support for the missing controls.