Keep the syncer even in an error status. Don't init_sync in force_sync,
since we already have a syncer.

When clicking the "Open local folder" option on the GUI menu, the
parent folder would open instead, in some platforms. With this fix
we have the expected behavior in all platforms.

When starting GUI from CLI, the user may stop it with a keyboard
interrupt. We treat this as equivalent to quitting the application
through the GUI, therefore the interrupt is caught and handled
accordingly.

The implementation required to move some launching code from
"gui.py" to "cli.py", which enabled the reuse of some CLI
methods (_start and _shutdown).

While initializing the UI daemon, syncer errors would be forwarded
to UI clients as part of the client response. This confuses the
clients. Instead of forwarding the error, we now understand it and
change the UI daemon status accordingly.