LibGUI: Add GColumnsView

This is a shiny new widget that can display a tree using Miller columns ^:)

In many cases, the columns view can be used as an alternative to tree view,
but it has its own set of limitations:

* It can only display one model column (so it cannot replace a table)
* It takes up a lot of horizontal space, so it's only suitable if the item text
  is fairly short
* It can only display one subtree at a time

But as long as a usecase doesn't suffer from these limitations, a columns view
can be *much* more intuitive than a tree view.

Libraries/LibGUI/Makefile

@@ -41,6 +41,7 @@ OBJS = \
     GTreeView.o \
     GFileSystemModel.o \
     GFilePicker.o \
+    GColumnsView.o \
     GSplitter.o \
     GSpinBox.o \
     GGroupBox.o \