Make startup notification work for MicroB menu entry

To have working startup notification, we have to provide a D-Bus service
with a top_application method; hildon-desktop will then use that to
launch the application instead of invoking the binary directly.  Co-opt
the org.maemo.garage.browser_switchboard name previously used for
locking for this (replacing a - with a _ to get around D-Bus naming
restrictions).

While we're at it, remove the non-standard switchboard_launch_microb
method from the com.nokia.osso_browser interface we implement, instead
providing a launch_microb method in our own interface.