The Great DataSource Refactoring

[davidrg]

The current data layer could at best be described as a mess. Its entirely synchronous and runs on the main thread which causes problems for reports (#27) and results in weird side-effects like the WebDataSource feeling faster than the DatabaseDataSource even though its actually slower (it just doesn't lock up the UI due to being async internally).

This work needs to be done before The Great Chart Refactoring (#42) can happen. This work is independent of #27 - even after the data sources are refactored/rewritten it may still be preferable to run reports in a subprocess and there are other drivers for refactoring the data source code into libraries than report subprocesses.

There are design notes for how the new data sources should be structured on onedrive but the work will be a bit disruptive so shouldn't be tackled for v1.0 - that's been delayed for long enough already.