Add ARIA support to Mobile Menu/Offcanvas toggle for improved accessibility

[N8Solutions]

In reviewing Gantry 5 for accessibility enhancements I noticed the Mobile Menu / Offcanvas Toggle has no ARIA attributes associated with it.

I do not believe aria-hidden should be added to the icon since it is an Interactive Element nor should it only be recognized as a menu because it can also be set to be visible on tablets and larger screens that may not contain menu items.

I would suggest the ARIA support be added in a way so that it recognizes the Toggle as the Menu when the screen size is that of a mobile and something like Expand Offcanvas.

I found this issue on GitHub from Automattic and how they addressed the issue of accessibility with the Toggle.
Automattic/_s#545

[mahagr]

Automattic/_s@e142af3

Great idea. I'm assigning this to @w00fz as there's javascript change which needs to be done.

[mahagr]

By default the particle div should have: aria-controls="menu" aria-expanded="false". When mobile menu gets shown, javascript should change aria-expanded="true" and back to false depending on the state of the menu.

[N8Solutions]

@w00fz You're awesome Djamil! Thank you for taking care of this!