Update grid system

## What this does
Implements a set of dynamic placeholders for a custom grid system.

The placeholders include a default row (`%grid-row`) and a default set of columns (`%grid-column-1` to `%grid-column-12').

It also includes a set of responsive columns for different scenarios, e.g extending `%grid-column-large-4` and `%grid-column-medium-8` is going to render a 4 column element on a large screen and an 8 column element on a medium resolution.

The new set of global variables allow setting up a custom placeholder prefix _(the default is `grid-column` )_ and a custom set of screen sizes _(default are `small`, `medium` and `large`. Add xlarge and xxlarge if required for example)_.

The plan is to improve this grid generator by adding offsets, centering and so on. Right now there's no use for these features, that's the reason for not implementing them.
Just to clarify: that rule of course assumed a row didn't have any other children except for its columns.