[TASK] Move code for clearing text fields to jquery plugin

The code for clearing text fields in the extension manager is now
encapsulated in a jquery plugin, and the form for searching the TER
is automatically submitted if it is cleared and a search result was
shown.

Resolves: #46853
Related: #43410
Releases: 6.1
Change-Id: I36ec6b3614cf18551018eba1d0dee15bdaf63b5e
Reviewed-on: https://review.typo3.org/19505
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn