[model] Rework Model behaviour with a filter
Currently ClutterModel::get_iter_at_row() ignores whether we have
a filter in place. This also extends to the get_n_rows() method.
The more consistent, more intuitive and surely more correct way to
handle a Model with a filter in place is to take into account the
presence of the filter itself -- that is:
- get_n_rows() should take into account the filter and return the
number of *filtered* rows
- get_iter_at_row() should also take the filter into account and
get the first non-filtered row
These two changes make the ClutterModel with a filter function
behave like a subset of the original Model without a filter in
place.
For instance, given a model with three rows:
- [row 0] <does not match filter>
- [row 1] <matches filter>
- [row 2] <matches filter>
- [row 3] <does not match filter>
The get_n_rows() method will return "2", since only two rows will
match the filter; the get_first_iter() method will ask for the
zero-eth row, which will return an iterator pointing to the contents
of row 1 (but the :row property of the iterator will be set to 0);
the get_last_iter() method will ask for the last row, which will
return an iterator pointing to the contents of row 2 (but the :row
property of the iterator will be set to 1).
This changes will hopefully make the Model API more consistent
in its usage whether there is a filter in place or not.