As the UI doesn't have semantic groups for owner/group/others settings
but they are only visually apparent, add some more context to the
accessible version of the labels to identify what it targets.

This is especially useful for the setting for other's permissions,
because the focus never lands on the line with the "Others" label,
hence making it hard to infer the context while navigating.

Another solution would be to redesign the UI to have semantic groups,
like e.g. labeled frames, but it's a lot less trivial and a lot more
intrusive a change.

Neither the button nor the icon shown in the properties dialog had nay
name, which is a problem for accessibility as an assistive technology
thus doesn't have anything to present to the user apart from an unnamed
button or image -- which isn't particularly helpful.

Improve this in two ways:
* Add a description to the icon, either matching the file type display
  name of the icon, the fact it's a custom icon or conveying multiple
  files are selected and the icon is hence not very specific.
* Add a tooltip to the button hinting that it can be used to change
  the associated icon.

*In the wayland session, we now have mate-settings-daemon with GDK_BACKEND=x11
*We need it anyway to control theming in xwayland apps,so we can now use mate-appearance-properties to set the desktop background when it is running

* Wayland-background-dialog: fix memory leaks

* Wayland-background-dialog: check that the image is valid before using it

*Reduce the scope of the preview box

*Do not read the same value from gsettings before reading it from the color style combobox

Move image and colors labels, filebutton to left side of dialog