LibGUI: Add a GUI::Tray widget for the FilePicker common locations

The FilePicker has implemented its common locations tray as a composite
widget built from a GUI::Frame with a bunch of GUI::Button inside it.
The problem with that is that it creates a long and annoying chain of
keyboard-focusable widgets.

This patch adds GUI::Tray, which is a dedicated single widget that
implements the same UI element, but without child widgets.

Userland/Libraries/LibGUI/CMakeLists.txt

@@ -98,6 +98,7 @@ set(SOURCES
     TextEditor.cpp
     Toolbar.cpp
     ToolbarContainer.cpp
+    Tray.cpp
     TreeView.cpp
     UndoStack.cpp
     ValueSlider.cpp