set default action when trash applet activated

Summary:
--provide a default action to be executed when
the trash applet is activated

Test Plan:
--trigger the activated signal for the applet and see
if the trash:/ folder opens
--check that all the rest behavior of the applet isnt
influenced

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9133