[BUGFIX] Streamline "collapse" icon in form editor tree

The icon used to collapse / expand a node in the form editor tree is now
streamlined with the other trees (SVG tree, "static" tree).

Resolves: #100648
Releases: main
Change-Id: I09ffa90996ba277c1f7dc72694d3b991a3e01c24
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78715


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Jasmina Ließmann <minapokhalo+typo3@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Jasmina Ließmann <minapokhalo+typo3@gmail.com>