Fix some QoL issues introduced by #19

[setzer22]

Changes in #19 introduced a few new edge cases that need to be accounted for:

• Right now, mouse controls for graph panning (middle mouse drag) are handled everywhere. We should restrict that to be inside the graph's Ui.
• The 'new node' dropdown is not clipped to the graph's ui contents.
• The 'new node' dropdown sometimes draws before the window containing the graph, making it invisible.

None of this affects the original mode of operation, where the graph was being drawn as a full-screen CentralPanel.

[gmorenz]

The 'new node' dropdown is not clipped to the graph's ui contents.

I'm not sure if this might violate assumptions in egui, but I prefer the menu not clipping to the ui. From a consistency standpoint that's how menus usually work (see blender, for instance). From a user standpoint, if they're seeing that menu it's almost always because they are currently using it, so giving it screen real-estate is worthwhile and won't cover something else that they're trying to interact with.

[setzer22]

@gmorenz I think you may be right 🤔 I was just playing around in blender and I the same thing never bothered me there. The other two are still issues that need solving.

[setzer22]

Pull request #40 is going to fix these issues.