Improve memory usage for image assets

When a project is opened, all image assets are fully decompressed into memory, even if:

• The images are not used in any QML documents
• Qt Design Studio's "Assets" view is closed

This can cause extreme RAM usage in large projects. A customer reported Qt Design Studio taking up 16 GB of RAM while working on a blank *.ui.qml file, simply because their project contains many image assets.

Suggestions

• Don't load the images into memory unless a loaded QML document uses the image, or unless the "Assets" view is open.
• For the "Assets" view thumbnails: Instead of loading the full image into memory, load a cached thumbnail image. Or, set sensible limits the Image.sourceSize property of the thumbnails.