Use the TREE_GRID, ROW, COLUMN_HEADER and GRID_CELL roles
for the various widgets involved in a GtkColumnView. To
enable this, we subclass GtkListView for the internal
list in the column view.
Just always tell the title and cell widgets to
clip their children to the right size. Otherwise
we risk things getting out of sync and unintended
overdraw.
Fixes: #3179
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.
