fix: honor back/next state on page drops correctly

[marionebl]

This fixes a number of bugs around page drag/drop.

Main change

1. Decided based on DropState.next vs DropState.back if the Page should be inserted after (movePageAfter) or before (movePageBefore).
   Implementing the case handling exposed another bug where dropping/dragging a page on itself was not handled. I changed it to not show the drop indicators in this case.

Yak shaving

2. Remove project.reArrangePagesIndex, use project.movePageTo instead
3. Abstract project.movePageAfter to project.movePageTo to power project.movePageAfter and project.movePageBefore
4. Provide and use getIndex on Page to avoid intermediary states in PageListContainer. Only state in PageListContainer now is draggedPage.