[TASK] Remove notification behaviour changes

see https://forge.typo3.org/issues/35245
[TASK] Remove grid nesting behaviour changes
[TASK] Remove PageTree JavaScript changes
[TASK] Remove stage handling and selection changes
[TASK] Remove custom preview page definition changes

see https://forge.typo3.org/issues/59144
[TASK] Remove superfluous record domain model
[TASK] Hook registration modifications
[TASK] Raise version to 0.6.0-dev