The button for expanding the find/replace overlay to show the replace
bar it quite large on MacOS and also changes its size upon different
events, such activating options and scrolling in the editor target.

With this change, the button style is changed to "FLAT". It results in a
less space-consuming layout and fixes its size so that it does not
resize on events like scrolling in the target editor.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/2018