Add a button to searchbar to replace current match only

While replacing a string one by one in a doc, clicking Replace, replaces the currently selected match, then automatically searches for the next match, if any, and select it; this works fine most of the time but occasionally one wants to examine the replacement before proceeding to the next one.

Mimic the UI from Qt Creator, which has two individual buttons, "Replace" and "Replace & Find".

Add unit test.