Revise column state attributes

[bmblb]

Consider following usecase.

1. There's an app like this which saves its state:

import 'MyFilterField.js';

grid = new Grid({
  columns: [
    { type : 'mycolumn', filterable : { filterField : { type : 'myfilterfield' } } }
  ]
});

// load state if exists
const state = JSON.parse(localStorage.getItem('foo'));
if (state) { grid.state = state; }

// save state to localStorage
localStorage.setItem('foo', JSON.stringify(grid.state));

2. Now inspect local storage and see column has filterable with field type.
3. Let's say app changed and we no longer import MyFilterField:

grid = new Grid({
  columns : [
    { type : 'mycolumn' }
  ]
})

4. now when we load the app we get an exception, because column is trying to instantiate a widget which doesn't exist

To do:

1. filterable should not be part of state, it cannot be modified in UI
2. filters are saved in the store state, column has nothing to do with them

[isglass]

Agree, does not make sense to store filterable in state. Please also remove text while at it

Handled in Column.getState()