When opening the FindReplaceDialog with an active selection in a text
viewer, the input field for the find string is filled with the content
of that selection. In addition, the replace functionality of the dialog
is activated. Currently, using the replace functionality does not work
because it requires a find operation to be executed before, which is
currently not done.

With this change, initializing the dialog with the current selection
performs the missing find operation before executing a replace
operation. The according functionality is refactored and a regression
test is added.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/1754