The check for available slot in PluggableUSB is done on the endpoint
and not on the number of plugged modules.
The modulesCount field is no longer useful and it has been removed.
This change allows the compiler to handle callbacks resolution.
Callbacks now must be implemented on the class that extends
PUSBListNode and this is forced by compiler by means of pure
virtual methods.
Also the calls to HID.interface() and HID.endpoint() can now
be simplified to interface() and endpoint() respectively since
the methods are no more static.
The method
int8_t PluggableUSB::addFunction(PUSBListNode *, uint8_t *)
has been changed to
bool PluggableUSB::plug(PUSBListNode *node)
since both EP and Interfaces are now saved directly into node