LibGUI: Move GUI::SeparatorWidget from ToolBar.cpp to its own file

This makes this nice line separator widget usable outside toolbars.
Also support both horizontal and vertical orientation. :^)

Libraries/LibGUI/CMakeLists.txt

@@ -65,6 +65,7 @@ set(SOURCES
     RunningProcessesModel.cpp
     ScrollBar.cpp
     ScrollableWidget.cpp
+    SeparatorWidget.cpp
     ShellSyntaxHighlighter.cpp
     Shortcut.cpp
     Slider.cpp