This is a somewhat large commit that:
- Adds GtkColumnViewSorter
This is a special-purpose, private sorter implementation which sorts
according to multiple sorters, allowing each individual sorter to be
inverted. This will be used with clickable column view headers.
- Adds a read-only GtkColumnView::sorter property
The GtkColumnView creates a GtkColumnViewSorter at startup that it uses
for this property.
- Adds a writable GtkColumnViewColumn::sorter property
This allows defining per-column sorters. Whenever an application sets a
sorter for a column, the header becomes clickable and whenever
a header is clicked, that column's sorter is prepended to the list of
sorters, unless it is already the first sorter, in which case we invert
its order. No column can be in the list more than once.
This uses a custom GtkColumnViewTitle widget. So far that widget is
pretty boring, but that will change once we added
resizing, reordering, dnd, sorting, hiding/showing of columns or
whatever UIs we want.
The ColumnView now allocates column widths first and then the individual
rows use the new layout manager which looks at the column allocations to
allocate their children.
It's a GtkListItemWidget subclass that tracks the column it belongs to
and allows the column to track it.
We also use this subclass to implement sizing support so columns share
the same size and get resized in sync.
