Switch to crates.io dependency for egui, egui_graph, eframe #63

Pending a new release of egui (newer than 0.31.1) that includes:

• Scene: Set transform layer before calling user content emilk/egui#5884
• feat: Add Scene::drag_pan_buttons option. Allows specifying which pointer buttons pan the scene by dragging. emilk/egui#5892
• Add Scene::sense to customize how Scene responds to user input emilk/egui#5893

To-Do

• https://github.com/mitchmindtree/egui_graph/pull/29
• Switch to crates.io deps for egui, egui_graph, eframe