Added more content

Added reference to component repo and expanded the description a bit

Merge branch 'master' of https://github.com/fer-rum/q2d

• added Node::hasDriver() for convenience

• added SchematicElement::id() for convenience, due to this,
PortGraphicsItem::id() is removed
• one can now only drag a wire in data flow direction i.e. from a data
source (output port) to a data sink (input port)
• added metamodel::enums::ElementType::FUNCTION
• functions of a component are now displayed in the component descriptor
hierarchy
• elements in the component descriptor hierarchy now use the qt
ressource system
• since functions are mostly to long to fit in whole, a tool tip display
it all
• function items in the component hierarchy also got a nice icon

Added references and links to the readme