[FEATURE] Install Tool: Check for broken ext indicates loading

"Check for broken extensions" in important actions of the install
tool checks for broken extenions over an Ajax-Request.

Currently, the user clicks the button to check for broken extensions
and doesn't get any feedback whether there is something going on or not.
This patch implements a loading-message with a Spinner that is displayed
as soon as the button to check for broken extensions is clicked.
Furthermore the button is hidden as long as the request is beign executed
and displayed again if there is anything more to check (e.g. broken
extensions are found).

Resolves: #51379
Release: 6.2
Change-Id: I13e5024a1e2666e54e782ff1bab9418ba8b180f6
Reviewed-on: https://review.typo3.org/23365
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Christoph Buchli
Tested-by: Christoph Buchli
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters