Adapt_framework: add 'hide from menu' option

[moloko]

As is evidenced by this discussion we have no standard method for stopping a page from being displayed on the menu.

It would therefore be useful to add something to Adapt to handle this situation.

Suggest adding a property _hideFromMenu as a property of contentObject and then implementing this in boxmenu

[tomgreenfield]

Suggest using the already available _isHidden.

[moloko]

done in https://github.com/adaptlearning/adapt-contrib-boxmenu/commit/d3f942fdbdf2f69fd9bf2e3b59e7869c15a71f31 - using _isHidden instead of creating a new property, as per suggestion