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.