Blender Git Loki

File Browser/macOS: Don't treat .app as directory

While there are other bundles which show up as directories,
`.app` are the most common ones.

*Users should not be saving anything inside .app bundles, nor using
Blender to edit any of the files.
*This declutters the File Browser for say ~/Applications folder
or recursive search on a path with apps.
*Matches Finder's behavior of showing apps as files. (We don't have a
"right click > show package contents" button like Finder though)

This change shows `.app` files like incompatible files, or `.exe`s
on Windows.
{F8970986}

Reviewed By: #platform_macos, brecht, mont29

Differential Revision: https://developer.blender.org/D9162