[BUGFIX] Omit checkbox for unselectable tree items

[BUGFIX] Omit checkbox for unselectable tree items

TCA select fields with renderType = 'selectTree' have the option
treeConfig.appearance.nonSelectableLevels, which doesn't seem
to work anymore since TYPO3 v7. The items are correctly flagged
with node.selectable = false in PHP but the JavaScript doesn't
check for that property.

Resolves: #84708
Releases: master, 8.7
Change-Id: Iefe98e1c9fd316f05c27ad1230d63236a7069bf1
Reviewed-on: https://review.typo3.org/58306
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>