The FindReplaceOverlay currently uses the default focus order. When
tabbing through the widgets, this means that when starting from the find
input field you have to tab over all the search options before coming to
the replace input field. Since the most often used navigation is between
the two input fields for find and replace, they should come after each
other in the focus order.

This change adapts the focus order of the involved elements:
- Tab between find and replace input field
- Tab between replace input field and the tools (starting from the
search tools)
- Tab between search/close and replace search tools (instead of coming
from search/close tools to the replace input field)

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/2161