Fixes for issues #18, #19, #21, #22, #23, Migration to AndroidX and kotlin and updated all dependencies

[vyguera]

This pull request adds the following:

• Support for Android Studio 3.6.0. Update of Gradle to 5.6.4
• Migrated to Android X to update target and compile SDK to API Level 29
• Migrated code to Kotlin.
• Refactoring to clean up some repeated code.
• Fixes for issues #18, #19, #21, #22 & #23
• Fix for a bug that was showing the dragging drawable at the beginning of the layout (top or left sides for vertical and horizontal orientations) when user passed away the ending limit (bottom or right sides); on a screen that has some other view below/next to the SplitPaneLayout view. The expected is the splitter is clipped to the end.

Note: Please, ignore commits for issue #20 as this issue came to be absolutely unnecessary. They have been reverted in this same pull request.