class TransfersMenu(Dbusmenu.Menuitem) shouldn't be used if importing fails

[ciel]

/usr/lib/python2.7/dist-packages/ubuntuone-client/ubuntuone/platform/sync_menu/linux.py
---
try:
    from gi.repository import (
        Dbusmenu,
        SyncMenu,
    )
    use_syncmenu = True
except:
    use_syncmenu = False
---
When this fails, according to
> UbuntuOneSyncMenu = UbuntuOneSyncMenuLinux if use_syncmenu else DummySyncMenu
DummySyncMenu is used, but
> class TransfersMenu(Dbusmenu.Menuitem):
still depends on the constant Dbusmenu, causing ubuntuone-syncdaemon fail, resulting in IPCError on GUI.

There are situations where Dbusmenu constant is not available. so
> class TransfersMenu(object):
should be used instead.