Add slides before and after existing slides

In the current behaviour of the quiz editor, newly imported slides are always added at the end of the Quiz. This is very unnatural behaviour, and very difficult to manage if one slide needs to be inserted at the beginning of a large Quiz.

This merge request brings multiple ways to make this easier, as in line with #11 (closed):

• By using the already existing buttons, new slides will now be created directly after the current slide instead of at the end.
• By using the new slide options on slides in the slidebar (on mouseover a + dropdown appears) it is possible to add slides after a slide or before a slide. As an added bonus, this menu also has the possibility to delete a slide, and can also be opened by right-clicking the slide (context menu).

The way this is implemented is by turning each slide import into a batch task. First, a task is created that imports the slide(s), then, for each imported slide, a move task is added to the batch which moves the slides either before or after the chosen existing slide.