tests: Check for orphaned layout files

This adds a test which scans the layouts directory for files which have not been added to the resources file.
The majority os user-submitted layouts don't include layout registration, and I ran out of ideas for how to make it more visible in the documentation. Instead, it's going to be solved on the other side: the CI pipelines shall fail.

To make it easy on experimenters with untidy trees, it's disabled by default.

src/lib.rs

@@ -39,7 +39,7 @@ mod outputs;
 mod panel;
 mod popover;
 mod receiver;
-mod resources;
+pub mod resources;
 mod state;
 mod style;
 mod submission;