A follow up to 0faacaa7c8 to cover the stop button within the launcher
and terminating Syncthing on shutdown/exit. To find the relevant connection
the connection settings are searched for a local URL where the port matches
the port from the Syncthing process log.
* Use a process group / job object via Boost.Process to be able to
terminate sub processes as well
* Do not try to stop the process gracefully under Windows by posting
WM_CLOSE because this has no effect on Syncthing anyways
* See https://github.com/Martchus/syncthingtray/issues/94
So far the backend libraries' include paths were relative within this
repository. This means the header files could not be used at their
installed location.
This change replaces them with "<>" includes to fix that problem and adds
a new include directory so building everything at once still works.
With this change it should be easier to actually split some parts into
another repository if this one would become too big.