Save and load window state

[lawmurray]

Something very simple that I'm yet to look at is saving and loading the window state of the graphical application (i.e. saving its size and shape and keeping it for the next launch). For someone wanting to explore the code, this would be a nice first issue to look at.

Instructions on how to do this are in the GNOME Developer Documentation: Saving and Loading Window State. Mending Wall is already using GSettings, so the GSettings approach, as described there, should be taken.