Add Makefile for easier installation / uninstallation on KDE

[RobTheFiveNine]

This PR adds a Makefile that can be used by KDE users to manually install or uninstall the theme. The KDE installation requires a number of steps, which new users to KDE may not be aware of, so I thought this would be a useful thing to add in.

The install target, as can be seen in the instructions I've added to INSTALL.md requires a PLATFORM variable to be set to ensure that users will not run this in other desktop environments and assume it is supported. The file could be updated in the future though to add support for automating installation for other environments too though.