Fix the missing synchronization folder icon on Windows

The icon is setted via a special file Desktop.ini.

Issue

This file is created the first time the synchronization folder is created. Then, we only check for its existence and skip it.
If it was created with a version < 3.1, its content is something like:

[.ShellClassInfo]
IconResource=C:\Program Files (x86)\Nuxeo\Drive\appdata\nuxeo-drive-2.5.5.win32\icons\NuxeoDrive_Windows_Folder.ico,0
[ViewState]
Mode=
Vid=
FolderType=Generic

As the icon file no more exists, the icon disappears.

Fix

The icon is now always located at %LOCALAPPDATA%\Nuxeo Drive\folder_windows.ico.