Use LayeredIcon for mixin classes

[kikugie]

Motivation

Newly introduced mixin files icons look awkward - pixelation becomes noticable, they don't fit with the Intellij icon style and don't convey the information about the type of the class:

Solution

Intellij supports layered icons, similar to Kotlin classes, which are more visually pleasing:

New UI - dark

Old UI - dark

[kikugie]

Excuse the impatience, does this look good? (The old icon is irritating a bit)