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
Before this change syncthingwidgets unconditionally included the header
from libsyncthing so it couldn't be used as stand-alone library if
libsyncthing was disabled.
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.
* Lazy initialize systemd interface and don't initialize it
at all if the unit name is empty
* Allow to supervise/control system-wide units in addition to
user units (see https://github.com/Martchus/syncthingtray/issues/61)
* Avoid redundant code
This is useful because sometimes the background color of the system
tray is dark but the overall system UI still uses a light theme.
For the Plasmoid this should never be the case, though. Hence the
extra settings are not available there.
* Pass program and arguments directly
* Prevent failure on white space in executable path
* Use own parser for arguments
* Make libsyncthing accessible from launcher
Martchus