New plugin API: gs_plugin_add_recent()

[Richard Hughes <hughsient gmail com>]

Hi all,

I've just pushed a few patches that allow us to have a list of
recently released application on the overview page of gnome-software.
This of course needs help from the plugins to identify which plugins
have been recently updated.

I've added support for gs_plugin_add_recent() to the dummy, appstream
and flatpak plugins but I guess the snapd plugin needs this too. The
basic idea is that we return applications that are released sooner
than the number of seconds as specified in @age.

Any questions, yell. Thanks!

Richard.