Resolve "Login screen setup (without backend)"

Closes #19 (closed)

frontend/core/src/com/game/tankwars/controller/LoginController.java

+package com.game.tankwars.controller;
+
+import com.badlogic.gdx.scenes.scene2d.InputEvent;
+import com.badlogic.gdx.scenes.scene2d.InputListener;
+import com.badlogic.gdx.scenes.scene2d.Stage;
+import com.badlogic.gdx.scenes.scene2d.ui.TextButton;
+import com.badlogic.gdx.scenes.scene2d.ui.TextField;
+import com.badlogic.gdx.scenes.scene2d.utils.ClickListener;
+import com.game.tankwars.ResourceManager;
+import com.game.tankwars.TankWarsGame;
+import com.game.tankwars.view.GameScreen;
+
+/**
+ * Todo: Login user on backend
+ *
+ * Listens to the login button on the LoginScreen and logs in
+ * the user with the username provided in the username text field.
+ * Transitions to MainMenuScreen on login.
+ */
+public class LoginController {
+
+    private final TankWarsGame tankWarsGame;
+    private final Stage stage;
+    private final TextButton loginButton;
+    private final TextField usernameField;
+
+    public LoginController(final TankWarsGame tankWarsGame, final TextButton loginButton, final TextField usernameField, final Stage stage) {
+        this.tankWarsGame = tankWarsGame;
+        this.loginButton = loginButton;
+        this.usernameField = usernameField;
+        this.stage = stage;
+
+        setEventListeners();
+    }
+
+    public void setEventListeners() {
+        loginButton.addListener(new InputListener() {
+            @Override
+            public boolean touchDown(InputEvent event, float x, float y, int pointer, int button) {
+                handleInput(usernameField.getText());
+                return true;
+            }
+        });
+
+        usernameField.addListener(new InputListener() {
+            /**
+             * Unfocus text field and remove keyboard when enter is pressed,
+             * and move camera back to original position.
+             */
+            @Override
+            public boolean keyDown(InputEvent event, int keycode) {
+                super.keyDown(event, keycode);
+                if (keycode == 66) {
+                    usernameField.getOnscreenKeyboard().show(false);
+                    stage.unfocus(usernameField);
+                    stage.getViewport().setScreenY(0);
+                    stage.getViewport().apply();
+                }
+                return true;
+            }
+        });
+
+        usernameField.addListener(new ClickListener() {
+            /**
+             * Move camera down when text field is clicked
+             * to make the field appear above the keyboard.
+             */
+            @Override
+            public void clicked(InputEvent event, float x, float y) {
+                super.clicked(event, x, y);
+                stage.getViewport().setScreenY((int) (2 * stage.getHeight() / 3));
+                stage.getViewport().apply();
+            }
+        });
+    }
+
+    public void handleInput(String username) {
+        System.out.println(username);
+
+        // TODO: Move clear line to MainMenuController when the main menu is operational
+        ResourceManager.getInstance().clear();
+        tankWarsGame.setScreen(new GameScreen(tankWarsGame));
+    }
+
+}