Make "Web" menu entry and /usr/bin/browser open the default browser

Currently, no matter what the default browser is set to, the "Web" menu
entry opens MicroB, and running /usr/bin/browser does the same. This is
done to make sure that there's always a way of opening MicroB regardless
of the default browser setting.

However, this behavior would seem to be less than intuitive (why does
the entry marked "Web" not open the default browser? on Diablo, why do
the Web sidebar panel and Web menu entry do completely different
things?).  Therefore, do the following:

* Make the top_application method in the D-Bus interface open the
  default browser, not MicroB, and therefore make the "Web" menu entry
  open the default browser.
* Ship a new microb.desktop file which provides a MicroB menu entry that
  launches MicroB.
* Rename the existing /usr/bin/browser script to /usr/bin/microb.
* Ship a new /usr/bin/browser script which launches the default browser.

This has one primary disadvantage: none of the standard methods of
launching a browser on Maemo can be guaranteed to bring up MicroB (with
the old behavior, invoking /usr/bin/browser was guaranteed to bring up
MicroB, just as if Browser Switchboard weren't installed).