Added OSSettingsWriter interface

src/model/OSSettingsWriter.java

+package model;
+
+public interface OSSettingsWriter {
+    /**
+     * Sets the mouse pointer speed to a given new speed.
+     * @param speed The new speed of the mouse pointer from 1 to 20
+     * @return If setting the new mouse speed was successful.
+     */
+    boolean setMouseSpeed(int speed);
+}