Exclude introduction slides from batch actions for inserting statistics and explanation slides.

As described in #45 (closed), the Quiz Editor does insert extra explanation slides and statistics slides in the introductory block, the virtual question block 0. This does not make any sense as there is no source to generate new explanation slides from (since there is no actual question) and for statistics slide it would mean showing a slide without any information, as there is no statistics state at the introductory block.

This merge request fixes #45 (closed) by adding a feature to Slide to find the first Question block slide, instead of the very first slide. This is used inside getRange to get a range that starts at the first question block. In order to exclude the introductory slide in only 2 of the batch actions, the boolean parameter excludeIntro was added to getRange. This way the batch action only executes over the question blocks themselves, not over the introduction block.