[lipstick] Add a way so launch a .desktop using its Exec line

If HAVE_CONTENTACTION is defined LauncherItem::launchApplication()
will launch the application using libcontentaction, which means it
will use desktop files properties like X-Maemo-Service instead of
Exec to launch the application, if available.
Add a LauncherItem::execApplication() to always use the Exec line
instead.