main: Don't depend on GSystem unconditionally

We only need GSystem when running under systemd. As libgsystem itself
has a hard dependency on systemd, only import it when actually needed
to keep working on systems where systemd is not available.

https://bugzilla.gnome.org/show_bug.cgi?id=728449
workspaceThumbnails: Move to primary monitor first when creating workspace

When dragging a window preview from a non-primary monitor onto a
workspace thumbnail, the window is moved to the primery monitor
first before changing its workspace. However when dragging the window
between thumbnails to create a new workspace, it is kept on its current
monitor instead. This is not only inconsistent, but outright confusing
with the default 'workspaces-only-on-primary' setting, as the newly
created workspace is immediately removed again.

https://bugzilla.gnome.org/show_bug.cgi?id=683819