LibGUI: Add ControlBoxButton to LibGUI

ControlBoxButton consolidates the paint_event for buttons
used in composite box widgets like ComboBox and SpinBox. Its
button bitmaps are built with create_from_ascii like WindowFrame
and ScrollBar controls, making theming more uniform.

Libraries/LibGUI/CMakeLists.txt

@@ -15,6 +15,7 @@ set(SOURCES
     ColumnsView.cpp
     ComboBox.cpp
     Command.cpp
+    ControlBoxButton.cpp
     CppLexer.cpp
     CppSyntaxHighlighter.cpp
     Desktop.cpp