Completion: simplify render_proposal_{icon,text}_func()

The 'markup' column of a header row is now handled directly in the
CompletionModel.
Completion: simplify icon cell renderer

Instead of using a cell data func, add a boolean column in the model,
IS_HEADER, and use this column for setting the attributes of the icon
cell renderer.

It also moves all the init stuff of the TreeView in the new function
initialize_tree_view().
Completion: simplify text cell renderer

Same as for the icon cell renderer.

It uses only the MARCKUP column from the model.