When an output has the hotplug_mode_update property (e.g. SPICE/QXL
virtual displays), auto-configure to the new preferred mode on hotplug
events instead of applying stored configuration from monitors.xml.

This allows VM guest resolutions to follow host window resizes, which
stopped working when spice-vdagent switched from reconfiguring displays
directly to sending hotplug events expecting the DE to handle them.

Fixes #78

This replaces caja desktop detection with a generic check for
_NET_WM_WINDOW_TYPE_DESKTOP windows and makes the background plugin work
with any EWMH-compliant file manager.

It now treats the background plugin as a fallback: If there's no file
manager drawing the background, then m-s-d will draw it.

This also allows us to remove a bunch of unnecessary code:
- D-Bus session manager listener
- startup delays
- screen size tracking
- crossfade support (this is broken on msd anyway)
- checking show-desktop-icons gsettings (this is only used by caja)
- checking draw-background gsettings (this should be deprecated)

Fixes #335
Fixes mate-desktop/mate-session-manager#159

Remove the workaround that restarted mate-panel and marco when the HiDPI
scale changed. This is no longer necessary now that both components have
native support for dynamic scale changes.

The xsettings plugin now only manages the Gdk/WindowScalingFactor
XSetting itself. GTK applications receive the scale change through the
normal XSettings mechanism and update automatically via their
style-updated signal handlers.

This needs changes to mate-panel and marco to handle scale changes
dynamically:
- https://github.com/mate-desktop/mate-panel/pull/1521
- https://github.com/mate-desktop/marco/pull/814

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

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