Fix progress bar in the view message window:

Fix progress bar in the view message window:
   * A field sighandlers was added to ModestViewMsgWindow, to keep reference of the signal handlers using ModestSignalMgr.
   * The ModestViewMsgWindow is connected to the "queue-changed" signal from the operations queue. When an operation arrives, it attaches callbacks to its "operation-started" and "operation-finished" signals, and when it is removed the callbacks are disconnected.
   * Basically, the callback on_mail_operation_started() puts the toolbar in transfer mode, and on_mail_operation_finished() puts it back in normal mode if there aren't operations left.

pmo-trunk-r3795