[volume] add argument `$isDir` to access function

ex.
```php
/**
 * Simple function to demonstrate how to control file access using "accessControl" callback.
 * This method will disable accessing files/folders starting from  '.' (dot)
 *
 * @param  string    $attr   attribute name (read|write|locked|hidden)
 * @param  string    $path   file path relative to volume root directory started with directory separator
 * @param  object    $volume elFinder volume driver object
 * @param  bool|null $isDir  path is directory (true: directory, false: file, null: unknown)
 * @return bool|null
 **/
function access($attr, $path, $data, $volume, $isDir) {
	return strpos(basename($path), '.') === 0       // if file/folder begins with '.' (dot)
		? !($attr == 'read' || $attr == 'write')    // set read+write to false, other (locked+hidden) set to true
		:  null;                                    // else elFinder decide it itself
}
```
[volume] add check by `allowCreate()` on mkdir()
Merge branch '2.1' into 2.1_n