Be friendlier to external resources used by layouts, symbols, decorations, etc.

[nirvn]

ATM, if people have layouts, symbols, decorations, etc. referring to external files, they have to figure out to create an assets subfolder and add it into the additional attachments directory setting that dictates what extra folders will be packaged by QFC to serve projects to QField users.

It's obscure, and cumbersome.

I think we could easily improve things by relying on QgsProject path resolver's setPathPreprocessor(), whereas we could open a project, and capture all of the external resources being called when loading a project. We could then filter those by relative path to the project file, and package these automatically.

[suricactus]

This sounds as interesting and more robust check than https://github.com/opengisch/libqfieldsync/blob/master/libqfieldsync/project_checker.py#L113-L115