Upgrade flutter framework and dependencies, update/clean code, improve layout

android/app/src/main/res/mipmap-hdpi/ic_launcher.png

android/app/src/main/res/mipmap-mdpi/ic_launcher.png

android/app/src/main/res/mipmap-xhdpi/ic_launcher.png

android/app/src/main/res/mipmap-xxhdpi/ic_launcher.png

android/app/src/main/res/mipmap-xxxhdpi/ic_launcher.png

android/gradle.properties

assets/icons/button_back.png

assets/icons/button_start.png

assets/icons/game_fail.png

assets/icons/game_win.png

assets/icons/level_easy.png

assets/icons/level_hard.png

assets/icons/level_medium.png

assets/icons/level_nightmare.png

assets/icons/skin_default.png

assets/skins/default_empty.png

assets/skins/default_tile_0.png

assets/skins/default_tile_1.png

assets/skins/default_tile_2.png

assets/skins/default_tile_3.png