Profiler: Add fixed track headers to the timeline view

The architecture here is a little bit convoluted. I ended up making a
new container widget (TimelineContainer) that works similarly to
GUI::ScrollableContainerWidget but has two subwidgets (a fixed header
that only scrolls vertically, and the timeline view that scrolls on
both axes.)

It would be nice to generalize this mechanism eventually and move it
back into LibGUI, but for now let's go with a special widget for
Profiler so we can continue iterating on the GUI. :^)

Userland/DevTools/Profiler/TimelineHeader.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGUI/Frame.h>
+
+namespace Profiler {
+
+class Process;
+
+class TimelineHeader final : public GUI::Frame {
+    C_OBJECT(TimelineHeader);
+
+public:
+    virtual ~TimelineHeader();
+
+private:
+    TimelineHeader(Process const&);
+
+    virtual void paint_event(GUI::PaintEvent&) override;
+
+    Process const& m_process;
+    RefPtr<Gfx::Bitmap> m_icon;
+    String m_text;
+};
+
+}