Add commandline parameters to Backgrounds

[humdinger]

This is href44214.

It'd be nice to have scripting abilities for Backgrounds, i.e. commandline parameters. This could be the usage help page defining what would ideally be available:

Usage:

-g	--get					Return path/filename or RGB value
-l	--label-outline [on|off]		Set icon label outline on/off
-p	--placement [X:Y|center|scale|tile]	Set to X:Y coordinates (manual), or center, scale, tile mode
-s	--set [path/filename, RGB]		Set image or color value (e.g. #AA5F5F)
-w	--workspace [0-32|all|current]		Set affected workspace(s), comma-delimited, current or all

Examples:
Backgrounds --workspace all --set image001.jpg --placement 120:240
Backgrounds --workspace 1,2,4 --get