General code improvements (including ViewPager2)

[TacoTheDank]

1st commit

• Clean up .gitignore
• Delete unneeded files

2nd commit

• Update gradle wrapper to 6.7.1 using git bash

3rd commit

• Reorganize build.gradle to make a bit more sense

4th commit

• Update Android Studio 4.1.0 -> 4.1.1
• Replace deprecated kotlin-android-extensions

5th commit

• Remove unused libraries

6th commit

• Update buildTools 30.0.1 -> 30.0.3
• Update ConstraintLayout 2.0.2 -> 2.0.4 (changelog)
• Update Navigation 2.3.1 -> 2.3.2 (changelog)
• Update JUnit 4.13 -> 4.13.1 (changelog)

7th commit

• Utilize some KTX extensions

8th commit

• Migrate to ViewPager2
  • https://developer.android.com/training/animation/vp2-migration
  • https://developer.android.com/reference/androidx/viewpager2/widget/ViewPager2
  • https://medium.com/google-developer-experts/exploring-the-view-pager-2-86dbce06ff71

9th commit

• Some small code cleanup

Built and tested an APK. Everything works fine.

[ramzan]

Beautiful. Thanks a lot!