Forward Greeter accessibility settings to the GNOME session

Accessibility support is getting better in the new Greeter, but the settings chosen in the Greeter are not automatically forwarded to the GNOME session, so if one needs some accessibility tool, then they need to enable it twice. And if they need help from someone else to do so, then they need to request such help twice.

Relevant dconf settings, i.e. those one can modify from the Universal Access menu (there’s more one can fine-tune in the Universal Access settings panel but that’s irrelevant here as the user cannot do that in the Greeter):

• https://sources.debian.net/src/gsettings-desktop-schemas/3.22.0-1/schemas/org.gnome.desktop.a11y.applications.gschema.xml.in/
• https://sources.debian.net/src/gsettings-desktop-schemas/3.22.0-1/schemas/org.gnome.desktop.a11y.keyboard.gschema.xml.in/

Implementation resources:

• the dconf for administrators (reference doc)
• an answer with more practical approach

Parent Task: #14522

Related issues

• Related to #7500 (closed)

Original created by @alant on 12384 (Redmine)