LibGUI: Move file icon lookup into a separate FileIconProvider

Let's get it out of FileSystemModel so you can look up a nice icon for
a path without needing a model.

Libraries/LibGUI/Forward.h

@@ -45,6 +45,7 @@ class Frame;
 class GroupBox;
 class HorizontalBoxLayout;
 class HorizontalSlider;
+class Icon;
 class IconView;
 class JsonArrayModel;
 class KeyEvent;