The replace functionality of the FindAndReplaceLogic checks whether the
current selection fits to the current search string and otherwise
performs a find operation first. To this end, it compares the text
selection with the search string, even if regex search is used. In regex
search mode, this check will usually fail and a find operation is
performed even though a correct string is already selected. This results
in the replacement of the next instead of the current match.

With this change, the check for whether the currently selected string
fits to the search string properly considers regex search mode. In
consequence, all replace operations consider the current selection for
replacement if fitting. It also adds according regression tests.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/1540