Consider making filtering a first-class concept for Locators

[sdimitro]

Context: #184

The problem of the above PR raises an interesting question about a common design pattern for Locators. The PR also provides a workaround that could be considered good enough to be potentially reused (at the expense of a bit more conceptual complexity and boilerplate). We should at least think of potential alternatives to simplify this (including the title - making filtering a concept within the Locator class).