Keep selected and dragged nodes and edges inside `Graph` wrapper

[blitzarx1]

Graph wrapper for the moment is just wrapper with useful API. But it can be more as it is already a mutable state which leaves outside widget. We can simplify widget state storage where we use egui and use it only for metadata.
It will alspo be more relevant to keep selections and draggings inside Graph as they part of the graph state and not part of the widget state.