Export image opacity as property instead of baking it into the asset

Currently opacity of an image is always baked into the asset on export. Instead we should be able to export them as qml property (maybe even as a settings option?).

There are 2 ways of creating opacity in Figma: fill alpha and item opacity.
1. Fill alpha functions as a alpha channel of a fill texture, so in some cases we can translate it as alpha value for solid colors or gradients. But for complex things like images, we bake it into the image itself, as we should. 
2. Item opacity is similar to QML opacity, and for non-merged items, such as images, it should be preserved as customizable and dynamic property. 

With 3.1, we already translate Figma opacity to normal QML opacity for most items, even for vectors containers, and only Images are left out with baked in value, so it's also a question of consistency.