Filter values should have a value helper like the row editor

Enhancement request: in the Filter top bar, there should be a helper for default values, similar to the row editor (right side bar). This is kind-of implemented for the BOOLEAN type, but this is solved in the operator dropdown (logically, there are no other cases).

However, for especially date-columns, it is handy insert handy functions (I copied the drop-down in the screenshot from the row side bar). I could think of these for` timestamp`:

* `now()` Now
- `now()-make_interval(hours=>1`) One hour ago
- `now()+make_interval(hours=>1`) A hour from now
- `now()-make_interval(days=>1`) Yesterday
- `now()+make_interval(days=>1`) Tomorrow
- `now()+make_interval(months=>1`) Next month

These would be handy, because most of the time I have to look them up. Every Date/Time type needs some adjustment naturally.

## What software versions are you using?

Postico version: Version 1.5.8 (4819)

macOS version: macOS Mojave 10.14.5 (18F132)

PostgreSQL version: PostgreSQL 10.8 on x86_64-apple-darwin14.5.0, compiled by Apple LLVM version 7.0.0 (clang-700.1.76), 64-bit

Start by comparing the Filter top bar with the row editor and the existing BOOLEAN handling. Define how default-value helpers should work for date and timestamp columns, including the listed relative-time functions, and verify that the filter can use the resulting values.

Written by the indexing model from the issue text.