When enabling org.mate.interface window-scaling-factor-qt-sync (default), m-s-d will attempt to set QT_SCALE_FACTOR during the init phase.
When disabled, the user can then choose to control this env var elsewhere.

This change is a super hacky way of supporting the fallback desktop background (without caja) on HiDPI displays. It does this by doing two things: using a new function that scales up the background surface; and by loading settings using an updated function that triggers a redraw.

This changes how the media-keys manager determines successful
key-bindings from KeyRelease to KeyPress so that it does not conflict
with third-party applications that want to use modifier keys as global
key-bindings. We've already done this change for general keybindings and
marco.

It also cleans up a bunch of deprectation warnings.

Also fix build warnings on xsettings

Also set QT_* env variables during init, and restart marco, mate-panel, toggle desktop icons when changing scales

This change allows for Xft/Gdk settings to be changed dynamically for
the scaling factor and font DPI. It also has an optional auto-detection
algorithm for the most appropriate settings to use for different screen
sizes and resolutions. The auto-detect is re-triggered whenever the
screen size changes.

* Support font size and mouse cursor auto-scaling for HiDPI displays

* Fix where DPI is getting scaled

* Remove ceil for scaled DPI

Fixes build warnings

this requires https://github.com/mate-desktop/mate-desktop/commit/b133f518f6f2310093fd3b5248049c7048cfb8d4

closes https://github.com/mate-desktop/mate-desktop/issues/296