Extract common DataGridView and DataListView code into AbstractDataView

Even after recent refactors, DataGridView and DataListView still had
a huge amount of shared boilerplate that needed to be kept in sync.

This commit goes a step further by separating out everything common
into a new AbstractDataView component that both of them inherit from.
Now each one only contains code that is unique to it, which means that
the shared view layout and glue code only live in one place.