HackStudio+LibGUI: Implement matching curly brace highlighting

This works for C++ syntax highlighted text documents by caching the C++
token type in a new "arbitrary data" member of GTextDocumentSpan.

When the cursor is placed immediately before a '{' or immediately after
a '}', we highlight both of these brace buddies by changing their
corresponding spans to have a different background color.

..and spans can also now have a custom background color. :^)

Libraries/LibGUI/GTextEditor.h

@@ -120,6 +120,7 @@ protected:
     virtual void leave_event(CEvent&) override;
     virtual void context_menu_event(GContextMenuEvent&) override;
     virtual void resize_event(GResizeEvent&) override;
+    virtual void cursor_did_change() {}
 
     GTextPosition text_position_at(const Point&) const;