[BUGFIX] Extension manager styling

The extension manager never got all defined styles.
Bring several visual improvements:

— placeholder in search field
— show action icons on hover
— style extension state column
— float table header cols left and right

Change-Id: Id6eafa2b28b1d054929a84400bf1648b6e1e6d75
Fixes: #45928
Releases: 6.0, 6.1
Reviewed-on: https://review.typo3.org/18544
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Jost Baron
Tested-by: Jost Baron
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn