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
Closes #466
1 file changed