feat(offcanvas): add basic touch support

When ngTouch is available, if we detect a swipe of at least 30px,
we'll (de)activate the offcanvas menu, depending the side we have
swiped to.
feat(docs): allow touch support for the offcanvas

By adding ngTouch module, we can now have some basic swiping
to (de)activate the offcanvas.
This is since 0b9fec71910fc365cdb3fbc05e16266c1e7ba043.
fix(offcanvas): handle 2 or more instances properly

Previously, when 2 or more offcanvas instances were in the same
page, only the last of them would show up. This is because their
visibility was based in the .offcanvas element, which is body.