Replace dialog: Make search entry active

Depends on !7 (merged).

This makes the replace dialog search entry active, as is the case for the search bar: typing Enter or Shift-Enter triggers a search.

The difference here is that the search direction is set by the dropdown list.
Hence, the concepts of forward search and backward search are replaced by those of search and reverse search, that is "search in the direction set by the dropdown list" or "search in the reverse direction".