docs: add javadocs for nearly everything

src/main/java/eu/midnightdust/yaytris/Tetris.java

@@ -19,6 +19,13 @@ public class Tetris {
     private static TimerTask gravityTask;
     private static LocalTime startTime;
 
+    /**
+     * The application's entry point.
+     * Initializes the UI library to match the system style.
+     * Also loads saved settings, translations, and highscores from JSON.
+     *
+     * @param args   command line arguments – will be ignored
+     */
     public static void main(String[] args) {
         try {
             System.setProperty("java.awt.headless", "false");
@@ -32,10 +39,20 @@ public class Tetris {
         ui = new TetrisUI();
     }
 
+    /**
+     * Get the active game space
+     *
+     * @see          Space
+     */
     public static Space getSpace() {
         return space;
     }
 
+    /**
+     * Resets the game space, preparing it for a new game.
+     *
+     * @see          Space
+     */
     public static void resetSpace() {
         SoundUtil.stopMusic("/music/theme.wav");
         if (gravityTask != null) gravityTask.cancel();
@@ -43,6 +60,12 @@ public class Tetris {
         space = new Space();
     }
 
+    /**
+     * Starts a new game of Tetris :D
+     * This involves starting our gravity task, playing music and spawning the first {@link eu.midnightdust.yaytris.game.Tetromino}
+     *
+     * @see          Space#spawnTetromino()
+     */
     public static void startGame() {
         SoundUtil.playMusic("/music/theme.wav", true);
         space.spawnTetromino();
@@ -51,6 +74,13 @@ public class Tetris {
         timer.scheduleAtFixedRate(gravityTask, 1, Settings.difficulty.getTimerPeriod());
     }
 
+    /**
+     * Stops the current game.
+     * Disables falling, fades out music and handles saving of high scores.
+     *
+     * @see         ScoreMenu
+     * @see         HighScores
+     */
     public static void stopGame() {
         SoundUtil.stopMusic("/music/theme.wav");
         if (gravityTask != null) gravityTask.cancel();
@@ -60,13 +90,30 @@ public class Tetris {
         if (HighScores.scores.values().stream().noneMatch(hs -> hs > space.getScore())) ui.showHighscoreDialog(space.getScore());
     }
 
+    /**
+     * Updates the displayed score
+     *
+     * @param score  the new score
+     * @see          ScoreMenu
+     */
     public static void updateScore(int score) {
         if (ui.getMenuPanel() instanceof ScoreMenu) ((ScoreMenu) ui.getMenuPanel()).updateScore(score);
         updateLevel(score);
     }
+    /**
+     * Updates the elapsed time
+     *
+     * @see          ScoreMenu
+     */
     public static void updateTime() {
         if (ui.getMenuPanel() instanceof ScoreMenu) ((ScoreMenu) ui.getMenuPanel()).updateTime(startTime);
     }
+    /**
+     * Updates the displayed level
+     *
+     * @param score  the new score, from which the level will be calculated
+     * @see          ScoreMenu
+     */
     public static void updateLevel(int score) {
         int newLevel = Math.max(0, (int) (score / 1000f));
         if (newLevel != space.level) {
@@ -80,6 +127,9 @@ public class Tetris {
         }
     }
 
+    /**
+     * Defines our custom timer task that handles falling pieces.
+     */
     public static class GravityTimerTask extends TimerTask {
         @Override
         public void run() {