Have the icons of the Texture Bundle in the installer

I have attached a zip with the icons for the "Texture Bundle" of the Content Library – Textures and Environments.

Until now, if the user wanted to have textures in the Content Library, he had to download a big zip (about 1.48 GB) of textures from a certain location, save it locally, e.g. in `C:\dev\texture_bundle` (so he would have directories `C:\dev\texture_bundle\Textures` and `C:\dev\texture_bundle\Environments`), and probably have some configuration done for Qt Design Studio to know to look into the `C:\dev\texture_bundle` folder for textures.

With the recent changes, the textures will be downloaded automatically from the internet by Qt Design Studio, but we currently still require to have the "icons" (i.e. thumbnails) available locally. We don't want the user to have to separately download these icons – they are about 2MB in size, so they can safely be part of the Installer.