[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/19729
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
[BUGFIX] Images from non-local storages are not displayed

The content object renderer adds absRefPrefix to all
generated image URLs, leading to incorrect URLs for
storages not relative to the siteroot. Additionally,
the URLs generated by the driver are also escaped in
ContentObjectRenderer::cImage(), leading to broken
absolute URLs.

Change-Id: I40c0a7ea55ff3a1155e7a05b1fc37da7d0cf44e4
Resolves: #42223
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/15837
Reviewed-by: Alexander Opitz
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter
Reviewed-on: https://review.typo3.org/19730