Windows: Use the application icon for the sidebar

By setting the icon in Desktop.ini of the root folder, this adds the icon
both when browsing the folder directly and to the sidebar shortcut.

To avoid overwriting any user setting that could exist in Desktop.ini,
only do this if the file doesn't exist. Editing .ini files on Windows
isn't trivial and isn't worth it given that this file won't exist most
of the time.

Fixes #2446