LibGUI: Add AbstractZoomPanWidget widget :^)

This is an abstract widget that is meant to handle all the panning /
zooming functionality so that all applications implementing it do
not have to try to do their own coordinate math.

Userland/Libraries/LibGUI/CMakeLists.txt

@@ -9,6 +9,7 @@ set(SOURCES
     AbstractSlider.cpp
     AbstractTableView.cpp
     AbstractView.cpp
+    AbstractZoomPanWidget.cpp
     Action.cpp
     ActionGroup.cpp
     Application.cpp