gdp-hmi-launcher needs to handle an arbitrary list of applications

The launcher currently knows the unit names of all the applications up front. Adding a new demo application requires code changes to the gdp-hmi-launcher and the gdp-hmi-controller. A service could be added to gdp-hmi-controller to report back a list of installed applications. The launcher could then use this list to populate a menu of extra applications automatically. The list of apps would contain structures that would have a name, icon, unit name.