Lots of improvements. Better UI, time scrolling, scenario system

Scenarios/Scenario.cs

@@ -0,0 +1,23 @@
+using SFML.Graphics;
+
+namespace FluidSim.Core
+{
+    public abstract class Scenario
+    {
+        /// <summary>
+        /// Initialize the scenario with a given audio sample rate.
+        /// </summary>
+        public abstract void Initialize(int sampleRate);
+
+        /// <summary>
+        /// Advance one simulation step and return an audio sample.
+        /// The step size is 1 / sampleRate seconds.
+        /// </summary>
+        public abstract float Process();
+
+        /// <summary>
+        /// Draw the current simulation state onto the given SFML render target.
+        /// </summary>
+        public abstract void Draw(RenderWindow target);
+    }
+}