This allows the user to start typing and highlighting and jumping
to a match in ColumnsView, IconView, TableView and TreeView if
the model supports it.
Instead of SortingProxyModel having a column+order, we move that state
to AbstractView. When you click on a column header, the view tells the
model to resort the relevant column with the new order.
This is implemented in SortingProxyModel by simply walking all the
reified source/proxy mappings and resorting their row indexes.
We now create multiple levels of internal mappings between source
and proxy indexes, to support tree models.
This breaks selection update after resort, which we'll have to
deal with somehow.
This patch adds SortingProxyModel::less_than(ModelIndex, ModelIndex)
which is now used to implement the sort order. This allows you to
subclass SortingProxyModel if you want to tweak how sorting works.
Get rid of the awkward case sensitivity logic in SortingProxyModel
since that can now be done outside. Turns out nobody was actually
using it anyway, but it seems like something we will want to do
in the future someday.
