[BUGFIX] Remember selected page in fsMod and refactor page selection

Currently selected page is remembered in the fsMod js object.
The JS code responsible for handling click on the node text and node
background is refactored. Now both uses node.checked property.

Resolves: #85100
Resolves: #85106
Releases: master
Change-Id: I08768e82a62e942c247d968df707bf64426e1dab
Reviewed-on: https://review.typo3.org/57076


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>