Replace modified status icon.

[enlight]

The modified status icon has the same shape as the clean status icon, this is probably annoying for color blind people... and probably only slightly less annoying to those who aren't. The only reason the modified status icon has the same shape as the clean status icon is that I couldn't figure out which shape would make sense (seeing as the asterisk icon is already used by UnrealEd to indicate an asset with unsaved changes).