It's not very likely to have more modes of search besides normal and
trashed, but got surprised in that way quite often and it's nicer this
way anyways.
A new query/request parameter can be used to apply a search to only
soft-deleted items.
The query expression `Trashed` has been introduced which selects only
items with state `Deleted`. This is another option an analog to
`ValidItemStates` (both cannot be used together as they would select
no items). This new query node is not added to the parser, because
users may not use it in their own queries - it must be part of the
"fixed" query so the application can control in which subset to search
(it would otherwise be possible to select any items).
- put all used strings in one place to have it easier to track
- don't use `$` for shortcuts, it's a detail not interesting to a
user; now names must not clash (which is a good idea anyways)
- Added two more shortcuts `conc` and `corr`
