Roster: Take HTTPUpload into consideration when sending file via context menu

When sending a file via context menu, filetransfer_preference will be used to choose the preferred file transfer method. This enables file transfers to offline contacts as well (if httpupload is the preference). Fixes #9041 (closed).

For a http_upload file transfer, a chat window is opened (if not already open/switch to it if open) and the file chooser dialog gets shown. I tested this with both file transfer preferences, multiple ressources, single ressource, and offline contacts.