Adjust path to QtQuick3D assets in importer and component management

Currently imported 3D assets end up in ./assets_imports/QtQuick3DAssets.
The same is true for materials from the Content Library.

The QML engine team suggested to get rid of import paths and therefore our new project structure will only have a single import path (./).

The path for 3d assets then wil be simply ./QtQuick3DAssets.

Details and an example for the project structure can be found here: https://intranet.qt.io/display/QTRD/CMake+Generation.

Since we already adjusted the logic we can change QtQuick3DAssets to QtQuick3DComponents, since those are QML components and not just pure assets like images/mesh files etc.

./assets_imports/QtQuick3DAssets then becomes just ./QtQuick3DComponents.
The wizards will be adjusted by knud

This also applies to any other import under asset_imports like ComponentBundles.

Qt Design Studio has to be backward compatible with the old structure for old projects.

Currently generated under asset_imports:

• 3D imported components
• Components imported via content library (materials and effects)
• Effect composer effects

After some discsussion we decided to prefix als uris with: GeneratedComponents

e.g. GeneratedComponents. QtQuick3DComponents.

The name GeneratedComponents is still work in progress.