Add new data-alias attribute to make templates work with outside variables

[Dakror]

This PR adds the ability to use templates as reusable components within data-model trees. By wrapping the inline template in an element that defines variable name aliases using data-alias-<new-name>="<data address>", the template can refer to any outside variable by a fixed name.

[mikke89]

I'm not sure I understand the motivation. Why can't we just set the title data variable directly?

[Dakror]

How so? title is not a variable in the original data model. My use case is i define a component to render some form of inventory, but when using the component in different UI scenarios, the variable sourcing the inventory data might have different names

[mikke89]

Ah, I see. Does it also work as expected when including the template multiple times, possibly with different variables? That sounds pretty useful.

The implementation looks good to me.

Are you able to update the documentation with a small example? Thanks!
https://mikke89.github.io/RmlUiDoc/pages/data_bindings/views_and_controllers.html

[mikke89]

Looks good, thank you!