Interface which should be implemented in order to draw icons in the gutter area.
Gutter icons are drawn to the left of the folding area and can be used, for example,
to mark implemented or overridden methods.
Daemon code analyzer checks newly arrived gutter icon renderer against the old one and if they are equal, does not redraw the icon.
So it is highly advisable to override hashCode()/equals() methods to avoid icon flickering when old gutter renderer gets replaced with the new.
During indexing, methods are only invoked for renderers implementing
DumbAware.