Added keywords in org.kde.plasma-systemmonitor.desktop for common search terms especially when coming from Windows

So this is a relatively simple suggestion: When people like me with a Windows background want to start the system monitor they think of "task manager" and fail to find it using the search function when typing either "task" or "manager". This might be a user (me) issue but I've ran into this issue several times over the last months. My suggestion therefore is to add some Keywords for searching in org.kde.plasma-systemmonitor.desktop. I've added 5 terms that I think would be most likely to be used by users in need of the system monitor.