window sizing and file names

[paulfitz]

Some feedback from someone who used the app:

• It does not open to the same size at which it closed, so has the be resized each time if user needs that.
• The hack around file names (they are random and can't be moved) is annoying.

[SleepyLeslie]

These could be a part of the bigger plan to refresh Grist Desktop's UX (proposal WIP).

Auto-resizing window on startup is not natively supported by Electron, so we have to handle storing/reading the window size on our own. A sample implementation using electron-store: electron/electron#526 (comment)

File renames: Grist Desktop should not need to keep a central store of all documents like the server does. The config file could partially relieve this pain - now users can choose to store those .grist files somewhere distant.