maintenance release + bug fixes for ReduksFragment

kotlin -> 1.3.61
kotlin_coroutines -> 1.3.3
dokka -> 0.10.0
android gradle plugin -> 3.5.3
androidx.appcompat:appcompat -> 1.1.0
androidx.lifecycle:lifecycle-extensions -> 2.1.0
com.google.code.gson:gson -> 2.8.6
gradle -> 5.4.1

• improved async store tests
• introduced ReduksFragment abstract class for better integration of android fragments with reduks
• fixed method isFragmentVisible for ReduksFragment
• bugfix in reduksfragment, redukActivity was not defined as property method
• bugfix for ReduksFragment: attached activity can be null, if fragment not yet attached