Replace mate system monitor sidebar logo with ours

[fossfreedom]

https://discourse.ubuntubudgie.org/t/re-brand-mate-system-monitor/6711/5

Copyright as per the post.

Need to see if its possible to change the side.png via the package debian install scripts. Note - system monitor may not be installed at the point of budgie-desktop-environment package install normally - so we should add a recommendation in the control file for mate-system-monitor. i.e. recommendation to allow the end user to remove without forcing the removal of budgie-desktop-environment as well.

EDIT: Looks like we can just install a side.png in a distribution subfolder - doesn't look like ubuntu mate use that so we can use that ourselves

https://github.com/mate-desktop/mate-system-monitor/blob/e1533eb204f233e53e64ed86df636945da5cad6e/src/sysinfo.cpp#L926