Add an operator "Copy to Asset Library" for installing asset bundles into already-existing asset libraries. The operator is shown when: - the "Current File" library is selected, - the blend file name matches `*_bundle.blend`, and - the file is not already located in an asset library. The user can select a target asset library, then gets a "Save As" dialogue box to select where in that library the file should be saved. This allows for renaming, saving in a subdirectory, etc. The Asset Catalogs from the asset bundle are merged into the target asset library. The operator will refuse to run when external files are referenced. This is not done in its poll function, as it's quite an extensive operator (it loops over all ID datablocks). Reviewed by: Severin Differential Revision: https://developer.blender.org/D13312