Date limit for items in history instead of (or combined with) max number limit

- [ ] Add setting for `Expire items in history after days` to History settings tab.
- [ ] Add checkboxes to `Maximum number of items in history` and `Expire items in history after days` so those settings can be disabled.

I still run ditto alongside CopyQ because it can be configured to only remove entries after a specified number of days. This can be combined with a maximum limit although I leave that setting off so that I know I have a months worth (30 days) of clipboard entries.

I sometimes use the clipboard a lot, and would prefer not to lose recent items because I've hit an arbitrary limit, but don't want to have to set the maximum so high it impacts performance.