LibGUI: Add PathBreadcrumbbar

This Widget wraps both a Breadcrumbbar and a TextBox for editing the
path manually, based heavily on the existing code in FileManager.

Breadcrumbbar itself requires outside code to micro-manage what it does.
This class provides a simpler interface for it: Users don't have to
worry about segments, they just give/receive a string for the current
path.

Userland/Libraries/LibGUI/Forward.h

@@ -18,6 +18,7 @@ class Application;
 class AutocompleteBox;
 class AutocompleteProvider;
 class BoxLayout;
+class Breadcrumbbar;
 class Button;
 class CheckBox;
 class ComboBox;
@@ -56,6 +57,7 @@ class MultiView;
 class OpacitySlider;
 class PaintEvent;
 class Painter;
+class PathBreadcrumbbar;
 class PersistentHandle;
 class PersistentModelIndex;
 class RadioButton;