Fix xdg-mime for proper Plasma 5 support

I realized xdg-mime does not run kbuildsycoca5 when fiddling with my defaults causing Plasma 5 to use old defaults.

This adds checks for Plasma 5 and runs kbuildsycoca5 after updating mimeapps.list file. The path of the file is detected via qtpaths utility.

I've tested it with empty mimeapps.list and XDG_CONFIG_HOME env var set.