BE/FE templates: new option to enable/disable FE output of submenu left navigation

# Feature Request

THE PROBLEM
Using the templates available, I can get the left subnavigation menu only for some types of template,

THE SOLUTION
My idea is very simple. If you add a new option under PAGE PROPERTIES -> ASPECT
such as
enable/disable left menu
I avoid to create the double of templates or rearrange the existing templates.

ADVANTAGES
in this way I can use the existing templates, and I suppose you can find a solution to keep them available for backward compatibility.

More in detail, i.e., lets have a look to 50% 50% template
* this semplate doesn't allow to use the subnavigation left
* to use it I should modify the BE and FE template and create a new one

instead of:

* if I have an option to enable/disable subnavigation left menu is better