Add XSettings override gsettings to allow changing arbitrary XSettings
on GSettings without requiring code changes. This enables third-party
tools (like unity-gtk-module extensions) to integrate with MATE by
setting custom XSettings values at runtime.

The implementation adds an 'overrides' key to the xsettings plugin
schema which accepts a dictionary mapping XSetting names to values.

The complete list of XSettings is over at
https://www.freedesktop.org/wiki/Specifications/XSettingsRegistry/

Backported mostly from:
- https://gitlab.gnome.org/GNOME/gnome-settings-daemon/-/commit/c6e9079d
- https://gitlab.gnome.org/GNOME/gnome-settings-daemon/-/commit/35764838

Fixes: https://github.com/mate-desktop/mate-settings-daemon/issues/158

alt+super+s

Many of the plugins depend on mate_settings_plugin_get_type(), but this
symbol is only available at runtime in the mate-settings-daemon program.

With GNU libtool the -no-undefined flag gets silently ignored and the
build succeeds, but with slibtool it correctly sets -Wl,--no-undefined
in the LDFLAGS causing the build to fail.

To solve this simply removing -no-undefined is enough.

Gentoo issue: https://bugs.gentoo.org/922326

Many of the plugins depend on the libX11 and libXi linker flags, but
they are missing.

With GNU libtool the -no-undefined flag is silently ignored so the build
succeeds, but with slibtool is correctly sets -Wl,--no-undefined in the
linker flags.

Gentoo issue: https://bugs.gentoo.org/922326

The policykit-1 transitional package seems to no longer exist in Debian testing