Add GSetting for screenshot save directory.

Screenshots are now saved at the location specified by the setting org.gnome.shell.screenshot.save-directory, if it is set. Otherwise they will be saved in XDG pictures dir as is the current default.

This differs from !445 (closed) in two ways: it is implemented in the JS rather than C, and a new setting is created rather than assuming one from gnome-screenshot exists. Hopefully this addresses the reasons why that merge request was rejected.

There might be a localisation step I have missed in adding the schema for this setting -- I am new and haven't dealt with localisation before, and I was only able to test this code by setting $GNOME_SHELL_JS and manually compiling the schema rather than by building the whole of the shell.