Gridlayout

[matthijscox-asml]

Addresses issue #48 and should replace #58

I've decided to ignore GridLayoutBase.jl in favor of our own simple GridLayout type, since we do not dynamically update anything, we only resolve the grid sizes at write time.

The new docs shows an example of simple layouting.

Some considerations, which we might change in the future:

• No nested GridLayouts yet
• No overlapping grid cell allocation. You can always push 2 layouts into a Slide.
• Automatic resizing of the shapes to the grid cell span size, except Figures and Videos keep their ratio constant.
• Default padding and margins. Made the default top margin 40 mm, to avoid the title.