Add a read-more-clicked signal to GsAppWidget

We want different behaviour for the read more links in the
installed list and the updates list, so a signal makes sense.
Move the implementation for the installed list to gs-main.c.

Also, hide the box containing the button and spinner on the
updates list. This was reserving 200 pixels of empty space
 at the right.
Add signal in header
Implement read more link in the updates list