import on windows is dodgy

[paulfitz]

On Windows, importing is a bit wonky in a few ways.

Importing through the in-UI button results in a busy or locked error when unlinking a file (the import has actually succeeded at that point, if you reload, but all very confusing).

The File->Open menu doesn't work on windows, in various ways. Maybe best to hide it for now?

Opening a .grist file:

Opening a .csv file:

[paulfitz]

Latest version of the app includes some fixes that may help:

• Closes a file during import in a way that should make Windows happier.
• Tweaked symlinking (but may still not work for all users, Windows permissions around symlinking are complicated).
• Restricted File->Open to just .grist files for now since the code for importing is stale.
• Fixed importing via in-app button (part of the import code wasn't being packaged).