Avoid setting the mode twice at startup

The overview is already the default mode, so no need
to set it explicitly when --mode overview is passed
on the commandline.
Add a signal for 'refreshed' to GsShellOverview

This will let us avoid slowing down the loading of the overview
by deferring the load of the other tabs until the overview is
fully loaded.
Load updates and installed at start

We can avoid the wait most of the time, by loading the
updates and installed lists at start. But we don't want
to slow down the loading of the overview page (which is shown
by default), so we only load the others when the overview
is fully loaded.
Add a throbbing highlight style

This will be used to put emphasis on the updates button when
updates are available.
Put an emphasis on the updates button when there are updates

This uses the newly introduced throbbing style. We only
put the emphasis on the button if we are not already on
the updates page.