make gsettings path configureable (ie for running portably on windows, in debug, etc)

[0chroma]

GSETTINGS_SCHEMA_DIR

https://docs.gtk.org/gio/ctor.SettingsSchemaSource.new_from_directory.html

https://gnome.pages.gitlab.gnome.org/libsoup/gio/gio-GSettingsSchema-GSettingsSchemaSource.html