Rearrange file icons settings

This allows us to easily make the file icons settinsg insensitive when icons are disabled, or minimized apps icons is set. It also makes it clear that those settings only apply to file icons, and not other icon styles.

I've also moved the "Show 'Delete' option in file context menu" checkbox out of the desktop menu settings and into this new frame on the icons tab, as it's specific to file icons.