R1:428f9686c417

Composing a custom project picture: Remove misaligned empty icon

Summary:
Do not assume that all items under `/resources/builtin/projects/` are image files (some can be subdirectories). Only add PNG files to the map.

Could be nicer to use `is_file()` or `is_dir()` but we are dealing only with string representations of files and folders.

Closes T15696

Test Plan: Go to `/project/picture/1/` and click the `Choose Icon and Color...` button; check last icon shown under "Choose Icon".

Reviewers: O1 Blessed…