LibGUI: Add TreeViewModel as a simpler interface for building TreeViews

Having to subclass GUI::Model for even the simplest type of hand-built
TreeView makes them a bit unpleasant to work with at the moment. :^)

This adds such a GUI::Model subclass that is specifically designed for
adding nodes to a TreeView manually, supporting text and an optional
icon by default, and allowing for further data when subclassing the Node
class.

Userland/Libraries/LibGUI/CMakeLists.txt

@@ -106,6 +106,7 @@ set(SOURCES
     ToolbarContainer.cpp
     Tray.cpp
     TreeView.cpp
+    TreeViewModel.cpp
     UndoStack.cpp
     ValueSlider.cpp
     Variant.cpp