Sync-counter: data models

Topics: data models and view widgets

TODO:

• ListView and ListData
• Driver, including predefined impls
• Filter and UnsafeFilteredList. This rather messy to use (improvable?). The latter should eventually be replaced with a safe variant.
• MatrixView and MatrixData. (Will possibly gain support for row/column labels and be renamed TableView.)

For now, see the examples:

• examples/ldata-list-view.rs uses ListView with custom ListData and Driver
• examples/gallery.rs's filter_list uses UnsafeFilteredList with a custom Driver. Less code but possibly more complex.
• examples/times-tables.rs uses MatrixView with custom MatrixData and driver::NavView. Probably the easiest example.