ImageSelector\Dropdown::getList should return all files supported by current toolkit

Problem/Motivation

The file extensions to be used to retrieve the list of files in the dropdown image selector is hardcoded.

  protected function getList() {
    $filelist = array();
    if (is_dir($this->configuration['path']) && $handle = opendir($this->configuration['path'])) {
      while ($file = readdir($handle)) {
        if (preg_match("/\.gif|\.png|\.jpg|\.jpeg$/i", $file) == 1) { // @todo make this list dependent on toolkit capabilities
          $filelist[] = $file;
        }
      }
      closedir($handle);
    }
    return $filelist;
  }

Proposed resolution

Test files' extension against ImageFactory::getSupportedExtensions.

Remaining tasks

Write patch

User interface changes

List of image files will include any image supported by the default toolkit.

API changes

None

Data model changes

None