homescreenhandler: keep track of active apps

homescreenhandler: keep track of active apps

When the current app window is closed, the current behavior depends on
whether this action terminates the app:
- if the app is terminated, then we switch back to the launcher
- in the other case, we display the background, even if other apps are
  running

In order to implement a better behavior, we should keep track of active
apps and switch back to the previously active one when the current app
window is closed.

Bug-AGL: SPEC-4222
Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>
Change-Id: Idf1fe42886e95e2b37349b066204fde002d7c3b5