text: Title: Better handling of inverted icons
  Body: My thoughts related to #5450 - which is just one example of problems with icons in NC.
  
  I don't think using `icon-more-white` to display dark variant of the icon is a very elegant approach. I'm wondering if we can come up with a better solution for such situations ( #5450 is only an example). Here are several proposals that we might want to consider in future releases:
  
  1. SVG: use inline SVG as icon source (many pros and cons, I'm not going to list them here), set `fill` in CSS as needed
  2. JS: toggle between `icon-more` and `icon-more-white` classes using JS
  3. CSS: use CSS `filter: invert(100%)` property, which is quite nicely [supported](http://caniuse.com/#feat=css-filters) already
  
  My personal favourite of those is `fill`, but `filter()` would also be neat. Both of these would allow us to deprecate a considerable amount of existing icons. They would also give us possibility to theme icons (I guess folder icon is the only obvious candidate here).
  SVG-based solution would additionally allow us to have icons-within-icons without maintaining a separate asset, such as core/img/filetypes/folder-public.svg - but it might not be feasible just yet :cry: 
  
  I would like to hear your thoughts @nextcloud/designers @nextcloud/theming 