Better dataset editing, aggregation animations.

Main.js: load '.dataset.js' files automatically from datasets folder. Rename dataset files for this.
Add datasets/generator.js for generating simple data to test robustness.
DatasetEditorModal: allow choosing predefined datasets, allow generating of datasets.
NetworkGraph: better animations for aggregations, make the main svg absolutely positioned to prevent some scrolling.
Toolbar, store/data: add simulation of batch aggregations.
Workspace: reoder components, so graph is behind.
GraphNode, GraphLink: make cursor a pointer, add opacity prop.
NodePopover: hide attribute table for now.
Shared/Node: fix svg width.
Create helpers.js for shared functionality.