fix bug: assign jingle session id as sid in file_props, so that file transfer window gets updated correctly for the sender.

TODO: implement file transfer complete callback, various session management code for abnormal FT interactions.

fix coding style: use 'is' operator when comparing object to None. put space around '%' operator. remove redundant str() on arguments.

receive jingle file transfer session initiate message, shout 'FILE-REQUEST' event to connection object.

code added to trigger jingle ft session when needed

add jingle FT to gajim.gajim_optional_features

[eMerzh and me] ability to restart a new adhoc command without closing / reopening the window. Fixes #5698