Patch from tjyrinki (Timo Jyrinki)

Add InstantMessaging sub-category to desktop files

This was originally suggested by openSUSE's linter (to follow https://en.opensuse.org/openSUSE:Packaging_desktop_menu_categories), so I've carried it in my package but would prefer to have it upstream before I submit the package to be included in the distribution.

Differential Revision: https://phabricator.kde.org/D29737