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 Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15696

Differential Revision: https://we.phorge.it/D25499