I didn't realise that Cura already had a menu item. To prevent
Confusion, i've changed Cura.MenuItem to use UM.MenuItem and let all
other menus use Cura.MenuItem
As Ghostkeeper suspected correctly in the review comment https://github.com/Ultimaker/Cura/pull/9012#discussion_r549707433
the binding wasn't working because the model was being retrieved using a function
(CuraApplication.getFileProviderModel()).
Separating this model into a variable allows us to properly bind the "visible" properties of the
menu items with the count property of the model without a problem.
CURA-7868
When there is only one file provider (i.e. the local file provider), the Open File(s) will be a
simple item in the File menu.
When there are more than one file providers, the Open File(s) will become a submenu in the File
menu, which will contain all the file providers as submenu items.
CURA-7868
