FR: `gtrash summary`: support --trash dir, show latest timestamp for each trash can

[csswright]

2 feature requests after using gtrash for some time:

• Currently, gtrash summary shows summary of all trash cans (as advertised). But if calculating this status involves accessing all trash cans and potentially waking up spun down hard drives, then users might want to avoid this, limiting only to trash cans of interest at least for performance reasons. This is also the main reason I switched from gio trash which cannot restrict operations to a particular trash can and it would take ~5-10 seconds along with audible spin up of disks in my multi-drive system which also means wear and tear that could otherwise be avoided when possible.

This fits well with the rest of the app's usage which allows for this where possible.

• IMO, it's also useful for gtrash summary to show the most recent timestamp of a file trashed for each trash can which is arguably more useful than number of items. It gives more context on whether a trash can might be worth pruning and one might want a list of trash cans can then be sorted by most activity.