GridTester has no way to deselect a row, and no Grid editor support

Problem. GridTester has select(int), selectAll() and getSelected(),
but no way to undo a selection. In the browser a user deselects by
ctrl-clicking a selected row in multi-select mode, by clicking the row's
checkbox again, by unchecking the select-all header checkbox, or by clicking
the selected row in single-select mode.

var grid = new GridTester<>(component);
grid.select(0);
// no deselect(0), no deselectAll()

The Kotlin layer already has the pieces — Grid._select calls
deselectAll() internally for single-select before
selectionModel.selectFromClient(item).

While in there, two related gaps:

• Grid editor. Grid.getEditor() — the user double-clicks a row, edits the
  buffered fields and saves or cancels. There is no tester support for any of
  it. GridProTester.setValue covers GridPro's inline editing, but not the
  plain Grid editor.
• selectAll() doesn't consider
  setSelectAllCheckboxVisibility(HIDDEN), even though the Kotlin
  Grid._selectAll Javadoc claims it "fails if ... the select all checkbox is
  hidden". If the checkbox isn't rendered there is no way for the user to
  trigger it. (Worth deciding deliberately — this is the #168 question in
  reverse.)

Ask. Add deselect(int row) and deselectAll(), routed through the
selection model's from-client path like select(int) is. Decide on the
select-all-checkbox gate. Grid editor support can be split off if it's too big
for one PR.

Found while auditing the project against #153 / #168.

Start with GridTester.select(int), then compare the Kotlin Grid._select and Grid._selectAll paths and GridProTester.setValue for existing tester conventions. Decide the hidden select-all behavior and cover deselect(int), deselectAll(), and any included Grid editor support with tests; done means the client selection behavior and chosen checkbox rule are verified.