CSS-based vertical centreing - makes layout JS free

The centreing is based on a wrapper ```div``` which can be centred on the slide using CSS techniques. That wrapper is created during presentation initialisation in the ```setupDOM``` JS routine.

Furthermore, now the slides (i.e. the ```section``` CSS blocks) are fixed-size and well placed. This brings a clear distinction between placement/layout of elements on the slide and placement of the slide in the presentation universe (slide transitions/overview). It opens the path to more complex slide layouts, where e.g. the title is at the top but the rest of the contents are vertically centred, or slide footers, etc. 

This could possibly break user's ```section > some-content``` selectors, since there will be an intermediate div that is not visible to the user during the creation of the presentation. Such selectors should be replaced by ```.slide-content``` selectors, that class is added to the wrapper ```div``` just for that reason.