Clean up of app.h

app.h, generated from app.h_in, has unnecessary code duplication
and isn't a header file (it defines globals, static functions
and doesn't have a header guard, moreover, it has a 'using namespace
std;'). Because of this, a real headerfile that declares the stuff
defined in apps.h was missing leading to even more code duplication:
scattered forward declarations in .cpp files and an often repeated
type of std::map<std::string, px4_main_t>.

This patch moves cmake/qurt/apps.h_in to src/platforms/apps.cpp.in
(with some changes) and removes cmake/posix/apps.h_in.
Then src/platforms/apps.cpp.in is split into src/platforms/apps.cpp.in
and src/platforms/apps.h.in, splitting declarations from definitions.

A typedef is defined for the map (apps_map_type).

The main difference between cmake/posix/apps.h_in and
cmake/qurt/apps.h_in was that the first defined a global 'apps',
while qurt stores the apps in QShell. I opted to get rid of
the global variable (which are in general evil for various reasons)
and used the API of cmake/qurt/apps.h_in where a provided 'apps'
map is initialized with a call to init_app_map. Thus removing
the existing code duplication.