Add settings buttons

[MathiasVDA]

• Persist configuration
• Describe in readme the different settings

Settings should include:

• Straight vs curved arrows
• Filter by data type (classes, literals)
• Predicate display as label or icon (define different icons for commonly used predicates such as rdf:type, rdfs:label, skos:description, ... add at least 20 icons for the 20 most commonly used predicates)
• Think of at least 3 other useful settings to propose