LibGUI: Add IncrementalSearchBanner

Compared to traditional modal search, incremental search begins
matching as soon as the user starts typing, highlighting results
immediately. This refactors Itamar's work for HackStudio into a
common LibGUI widget to be used in all multi-line TextEditors.

Userland/Libraries/LibGUI/Forward.h

@@ -37,6 +37,7 @@ class HorizontalSlider;
 class Icon;
 class IconView;
 class ImageWidget;
+class IncrementalSearchBanner;
 class JsonArrayModel;
 class KeyEvent;
 class Label;