Improve styling of expand/collapse button in find/replace overlay #2018

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 #2018

bundles/org.eclipse.ui.workbench.texteditor/src/org/eclipse/ui/internal/findandreplace/overlay/FindReplaceOverlay.java

@@ -687,7 +687,7 @@ private void createMainContainer(final Composite parent) {
 	}
 
 	private void createReplaceToggle() {
-		replaceToggle = new Button(container, SWT.PUSH);
+		replaceToggle = new Button(container, SWT.FLAT | SWT.PUSH);
 		GridDataFactory.fillDefaults().grab(false, true).align(GridData.BEGINNING, GridData.FILL)
 				.applyTo(replaceToggle);
 		replaceToggle.setToolTipText(FindReplaceMessages.FindReplaceOverlay_replaceToggle_toolTip);