Change return of getExecutablePath() to fs::path
Remove execFullName from crash info

This was used to get the stack trace via gdb - gdb is smart enough to
figure out the executable from the PID only.

Also, the crash info already has the same info in executablePath.
Add getHelperExecutable() function

And replace equivalent code in CrashHandlerImpl.
Add an OS abstraction for running helper programs
Search for arx-install-data in the exe dir

In addition to $PATH.
Move getHelperExecutable() into platform namespace
Move getExecutablePath() into platform namespace
Move env_list_seperators into platform namespace
Move defineSystemDirectories() into platform ns
Move getSystemConfiguration() into platform ns
Move expandEnvironmentVariables() into platform ns
Move ExitStatus out of Environment.h
Remove unneeded #include
Cleanup platform/Dialog.h
Get rid of ARX_HAVE_WINAPI

We haven't supported Winelib builds in a long time, so there is no need
to make a distinction between Windows builds and non-Windows builds with
the WIN32 API available.

Replaced by ARX_PLATFORM == ARX_PLATFORM_WIN32 in source code and by
WIN32 in CMake code.
Change ARX_HAVE_PTHREADS to #cmakedefine01

Easier to catch typos that way.
Change ARX_HAVE_PRCTL to #cmakedefine01
Change ARX_HAVE_PTHREAD_SETNAME_NP to #cmake...01
Change ARX_HAVE_PTHREAD_SET_NAME_NP to #cmake...01
Change ARX_HAVE_OPENAL{,_EFX} to #cmakedefine01