Portal for housekeeping

## Problem

Currently housekeeping with flatpaks only effect trashed files, but that isn't the only thing housekeeping already has as a feature.

## Proposal

Create a portal for the system to housekeep apps' cache and tmp files under their name in the `~/.var/app/` directory. The user and the system already have access to those files; I image that developers could have some kind of flag either in the .desktop (or elsewhere) saying that their app can be housekept by providing a location of temporary files.
Of course there is the obvious 'nuke cache folder' option _(by completely emptying the pictured dir.)_, which could potentially be harmful, hence why it should be opt-in

To be fair, I'm not sure if this even needs to be a portal - could't there be an OS component that manually removes these files that are flagged as 'housekeepable'?

## Prior Art