LibWebView: Create a PropertyTableModel helper class for table models

We currently have StylePropertiesModel and AriaPropertiesStateModel in
LibWebView. These depend on GUI::Model and GUI::ModelIndex, which is the
only reason that the non-Serenity Ladybird chromes require LibGUI.
Further, these classes are very nearly idenitical.

This creates a PropertyTableModel to provide the base functionality for
all table-based model types used by all Ladybird chromes. It contains
code common to the style / ARIA table models, and handles the slight
differences between the two (namely, just the manner in which the values
are flattened into a list during construction).

The Qt and Serenity chromes can create thin wrappers around this class
to adapt its interface to their chrome-specific model classes (i.e.
QAbstractItemModel and GUI::Model).

Userland/Libraries/LibWebView/CMakeLists.txt

@@ -8,6 +8,7 @@ set(SOURCES
     Database.cpp
     DOMTreeModel.cpp
     History.cpp
+    PropertyTableModel.cpp
     RequestServerAdapter.cpp
     SearchEngine.cpp
     SourceHighlighter.cpp