Generic attributes for addresses

[jcelerier]

Maybe the API should allow for setting / getting custom attributes ?
For instance this would allow to add tags, description, etc... without too much hassle.

e.g. in Address :

virtual void setAttribute(const std::string&, Value*) = 0;
virtual Value* getAttribute(const std::string&, Value*) const = 0;

However not all protocols may be able to support this (e.g. MIDI) so it could be an extension, or have a default implementation :

virtual void setAttribute(const std::string&, Value*) {
    // do nothing
}
virtual Value* getAttribute(const std::string&, Value*) const {
    return nullptr; 
}

[jcelerier]

@thzebra this would be necessary in order to support all the attributes in the examples files

[thzebra]

ok, i'll see to it

[jcelerier]

done since a long time.