launcher: rely on applaunchd for application startup

In the new App FW setup, `launcher` should only instruct `applaunchd` to
execute an application. In order to do so, it must first build a list of
available applications by looking for and parsing `.desktop` files in
relevant folders.

Then, when an application must be started, it has to call the
corresponding `applaunchd` method through D-Bus, which will then handle
the application startup using either command line execution or D-Bus
activation.

Bug-AGL: SPEC-4159 SPEC-4160
Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>
Change-Id: Ie2f55a5acb64ed90aa6aafb687c927d87f6cc0aa