Make sure Plasma the DE is never referred to as KDE

Follow-up to !282 (merged).

I edited every instance where the Plasma DE was referred to as "KDE" - the group behind it; replaced by either the shorthand "Plasma" or the full name "KDE Plasma", in some cases where it was used multiple times I opted to mention the full name once followed by the shorthand.

I made sure not to touch other terms with "KDE" in it like "KDE developers" or "KDE's desktop environment".

Additionally:
Fix up GNOME capitalization in one case
Remove a couple extra whitespaces