Layout editor should open display layouts with missing or unknown imports

To improve the robustness of Designer's Layout editor, it should be able to open and edit layouts that might have missing or unknown imports to project (for instance, layouts with references for custom elements or layout blocks coming from other projects or dependencies).

This is an improvement to NXS-6493

AC

• I can load a layout that has a missing or unknown import
• A warning is shown saying "the referenced file [import file name] is missing: you may experience issues with this layout after deployment. We recommend to switch to code and remove this import."