global styling support

[michi42]

While the current look and feel of NodeGraphQt looks really good in a dark theme application, it feels like a foreign object in a light theme application.

Of course ideally one would inherit the style from the OS (#252) but for the time being, already being able to the default colors programmatically before creating a graph would be very useful. Unfortunately currently the defaults are a bit scattered among enums in constants and hard-coded constants in the qgraphics modules ... some of which can be customized per-instance and some can not (e.g. highlighted pipe colors) which makes it impossible to define a consistent custom color theme without monkey patching.

As a first step, it would be great if all color/them constants could be moved to a common place which could be overridden by the application, e.g. a Theme. Later this could possibly be constructed from QPalette to inherit the system theme...