Result category filters in search window

I think it would be great if we could add filter buttons for result categories or types.

We could limit this to only predefined types like "File", "Folder", "App", "Other",
But I think it should be doable to let plugins define their own categories, with the above examples being built used for the explorer plugin.

Perhaps this could work with the filters feature from everything - as requested in #2009

We may have to consider how to prioritise certain categories if we get too many, perhaps using a dropdown to see the rest.
The number of results is obviously a factor but perhaps there is some better approach

We could also consider filtering by plugin (where multiple use the same keyword e.g. *), particularly if letting plugins define their own categories gets too messy.

## Inspiration / Context
A user on reddit made a [post](https://www.reddit.com/r/FlowLauncher/comments/1uz70hn/category_tabs/) asking if flow could "display category tabs", and gave this screenshot:

Start by reviewing the existing filters feature requested in #2009 and how the search window handles result categories. The issue does not identify files or tests; before implementation, define whether categories are predefined or plugin-provided, how they are prioritized, and what completed filtering behavior should be.

Written by the indexing model from the issue text.