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