[BUGFIX] Pagetree pointer cursor broken since ExtJS upgrade

Icons in the pagetree should show cursor:pointer on hover.
This changed with ExtJS-upgrade in #52933 because of
x-unselectable.

Since in the pagetree we need x-unselectable unfortunately
manually bring back the old cursor-behaviour.

Change-Id: If6fa45b0e3491d9180855a4d0a462c5fb559d476
Resolves: #54238
Releases: 6.2
Reviewed-on: https://review.typo3.org/26099
Reviewed-by: Alexander Stehlik
Tested-by: Alexander Stehlik
Reviewed-by: Marcin S?gol
Tested-by: Marcin S?gol
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter