qdbusviewer: Restore the window size and state on startup

Save the window size, maximization state and state of splitters
on exit and restore these settings on startup.

Change-Id: Ibe26b09cc97ffc2fef17e6e8e2b804324dae7002
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>