Fix Action button icon

[DarkSide666]

Button class already takes care of icons so no need to (wrongly) do that again in ActionButtons class.

Consider example:

// create ActionButtons column without title
$column = $this->crud->table->addColumn(null, [ActionButtons::class, 'caption' => '']);

// see 1st parameter - seed for Button class, icon name passed as string
$column->addModal(['icon' => 'key'], 'Change Password', function ($v, $id) {...});

[DarkSide666]

@mvorisek approved in chat with some comments