Unify Frontend of PersonalAnalytics (using Electron)

[casaout]

Idea

• the backend (i.e. data trackers) for the Windows and Mac version are separate (and stay that for now)
• since the visualizations are already produced using JavaScript/D3 and visualized in a web wrapper, we'd like to unify the code of the UI to only have one codebase
• currently, the idea is to build an Electron app

Tasks

• [ ] Unify data access (API) on Mac (cc @csatterfield)
• [ ] think about useful abstraction
• [ ] create Electron-app
• [ ] migrate visualizations/views
• [ ] do the same for the other windows (e.g. Settings, FirstStartScreen, Credits)