drawLabel and drawHover inconsistencies

[jacomyal]

Here are some inconsistencies:

• drawLabel (for nodes and edges) and drawHover (for nodes only) could be static program class attributes, as they don't require anything from the program instances themselves
• They are actually required as non-static attributes for NodePrograms and EdgePrograms
• They are defined as static attributes of NodeImageProgram, but non-static for EdgeCurveProgram
• The getNodeImageProgram factory allows simply overriding the drawLabel and drawHover attributes, but the createEdgeCurveProgram doesn't (and their name don't fit either...)

So, actions to take:

• Check that all factories have similar names (getSomething or createSomething, but the same everywhere)
• Check that all factories allow simply overriding drawLabel (and drawHover for nodes)
• Clarify everywhere whether drawLabel (and drawHover for edges) should be static or not