[Grid]: Add ability to use selectable records

[Eugentis]

Now we have some strange example about selectable grid records - with grid description via big js array.

It is not comfortable to work with it in normal projects.

It will be more comfortable to have some options in GridSchema and use it on js-side.

As an example it can be smth like:

class SomeGrid extends GridSchema
{
    public function __construct() {
        $this->setOptions(
            [
                'selectable' => [
                    'type' => 'multi', // can be some const from toolkit
                    'id' => 'id',
                ]
            ]
        );
    ...

As I can see from toolkit code js already works with grid option:

if (this.options.selectable) ...

Additionally it should be described in doc with more adequate example