[#147,widget/filtereditor][m]: add filter support in filter widget plus move to sidebar and tidy up.

* model.js: xsmall tweak to set value of term filter to empty string rather than null.

test/widget.filtereditor.test.js

@@ -0,0 +1,26 @@
+module("Widget - Filter Editor");
+
+test('basics', function () {
+  var dataset = Fixture.getDataset();
+  var view = new recline.View.FilterEditor({
+    model: dataset
+  });
+  $('.fixtures').append(view.el);
+  assertPresent('.js-add-filter', view.elSidebar);
+  var $addForm = view.el.find('form.js-add');
+  ok(!$addForm.is(":visible"));
+  view.el.find('.js-add-filter').click();
+  ok(!view.el.find('.js-add-filter').is(":visible"));
+  ok($addForm.is(":visible"));
+
+  $addForm.find('select.fields').val('country');
+  $addForm.submit();
+  ok(!$addForm.is(":visible"));
+  $editForm = view.el.find('form.js-edit');
+  equal($editForm.find('.filter-term').length, 1)
+
+  equal(_.keys(dataset.queryState.attributes.filters[0].term)[0], 'country');
+
+  view.remove();
+});
+