Improve graphics

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_show_conflicts.png

assets/icons/button_start.png

assets/icons/difficulty_easy.png

assets/icons/difficulty_hard.png

assets/icons/difficulty_medium.png

assets/icons/size_2x2.png

assets/icons/size_3x2.png

assets/icons/size_3x3.png

assets/icons/skin_default.png

assets/icons/skin_food.png

assets/icons/skin_nature.png

icons/button_back.svg

icons/button_show_conflicts.svg