bus: Own a name on the DBus session bus
bus: Own a name on the DBus session bus

This makes it easier for third parties to track the ibus-daemon life
cycle.

BUG=http://code.google.com/p/ibus/issues/detail?id=1476
TEST=

Review URL: https://codereview.appspot.com/6374058
Patch from Rui Matos <tiagomatos@gmail.com>.

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