[BUGFIX] EM: add visual feedback for download+install

Process of downloading and installing remote extensions takes
some time - this patch brings visual feedback to inform users
that something complex is happening in the background.

Change-Id: Iae6c8ca90f163b49b0d6a63740eb773f29127d91
Fixes: #46274
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/19722
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn