mainwindow: fix url encoding issue

It appends "file:" before the input url if the url contains those chars
"@[^@]+$". It leads to fail to login to remote desktop, since the username
will be interpreted the wrong way.
ex. username@xxx.xxx.xxx.xxx becomes file:username@xxx.xxx.xxx.xxx

Signed-off-by: AceLan Kao <acelan@acelan.idv.tw>