LibGUI: add a rudimentary framework for Wizards.

This patch provides the basic components needed for developers to create
consistent wizard interface experiences in their applications.
`WizardDialog` provides the dialog frame for the wizard, handling navigation
and presentation.
`AbstractWizardPage`s form the base class of Wizard pages, which are
pushed onto the `WizardDialog` page stack via `WizardDialog::push_page`.
`CoverWizardPage` and `WizardPage` are provided to ease the creation of
Wizard interfaces consistent with the Serenity visual language.

Userland/Libraries/LibGUI/CMakeLists.txt

@@ -97,6 +97,10 @@ set(SOURCES
     Widget.cpp
     Window.cpp
     WindowServerConnection.cpp
+    Wizards/WizardDialog.cpp
+    Wizards/AbstractWizardPage.cpp
+    Wizards/CoverWizardPage.cpp
+    Wizards/WizardPage.cpp
 )
 
 set(GENERATED_SOURCES