treeview: Remove the pixel cache

This has caused numerous issues for users, especially in the filechooser, which have not been fixed in all the years since the pixel cache has been introduced.

If anyone seriously has complaints about the treeview performance (and those did not exist with the pixel cache), feel free to revert this commit and fix the pixel cache issues.

Closes #503 Closes #1691 (closed) Closes #466 (closed)