Fix issue #372

Several changes were made to fix the encoding of ftp dirpaths and allow
uploads to folders with non-ascii characters:

- [x] Fixed encoding of valid urls in ShareX.HelpersLib.NameParser.cs
- [x] Fixed subfolder path generation and wrong url re-encoding in
FTPAccount.cs
- [x] Fixed preview paths in FTP Destination settings (FTPAccount.cs)
- [x] Fixed subfolder path generation in FTP.cs

This fixes issue #372, which was reproduced with the following account
config on a local ftp server (import from clipboard on destination
settings):

```json
{
"Protocol": "FTP",
"Name": "New account",
"Host": "localhost",
"Port": 21,
"Username": "test",
"Password": "test",
"ServerProtocol": "ftp",
"SubFolderPath": "DEV/grhh本人%",
"BrowserProtocol": "http",
"HttpHomePath": "",
"HttpHomePathAutoAddSubFolderPath": true,
"HttpHomePathNoExtension": false,
"IsActive": false,
"FTPSEncryption": "Explicit",
"FTPSCertificateLocation": "",
"Passphrase": null,
"Keypath": null
}
```