use autocomplete widget for course category selector

testing instructions: test 1 (edit course settings) edit course settings check that course category field uses autocomplete widget change course category field and save form check that category has been changed test 2 (request course creation) request a course (<url>/course/request.php) check that course category field uses autocomplete widget test 3 (upload courses) go to site administration > courses > upload courses upload a csv file to access to the second step check that course category field uses autocomplete widget test 4 (edit database enrol settings) go to site administration > plugins > enrolments > manage enrol plugins edit external database settings check that course category field uses autocomplete widget actual result: course category field uses default drop-down list ( select tag in HTML) expected result: course category field uses autocomplete widget, with possibility to search some text in drop-down list