fpdfsdk/pwl is a Widget Library for AcroForms.
CPWL_Wnd is the base class that widget classes extend. The derived widget
classes are controllers for each widget. The hierarchy is:
* CPWL_Wnd
* CPWL_Button
* CPWL_CheckBox
* CPWL_PushButton
* CPWL_RadioButton
* CPWL_Caret
* CPWL_EditCtrl
* CPWL_Edit
* CPWL_Icon
* CPWL_ListBox
* CPWL_CBListBox (combo box)
* CPWL_ScrollBar
Widgets are rendered to Appearance Streams, with the case all centralized in
CPWL_AppStream.