If there's a configured and local Syncthing connection and we're on a
non-UNIX platform which doesn't support SIGTERM (basically Windows) it
makes sense to use the REST-API instead. That's likely better than just
terminating the process forcefully.
This doesn't cover the stop button within the launcher settings yet because
from this context is isn't clear which connection is relevant as there can
be multiple tray icons/widgets but only one settings page.
Otherwise `QMetaMethod::invoke` wouldn't be able to handle it:
```
QMetaMethod::invoke: Unable to handle unregistered datatype 'QProcess::ProcessError'
```
* 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
* Pass program and arguments directly
* Prevent failure on white space in executable path
* Use own parser for arguments
* Make libsyncthing accessible from launcher
Add new SyncthingLauncher class which lauches Syncthing
under the hood via external SyncthingProcess or using
libsyncthing.
Note: Launching via libsyncthing is still experimental.
Martchus