v1.6.1: Support SwiftPM on Xcode11!

Improvements

• Add Swift Package Manager support by @robbiet480

Bugfixes

• Fix the crash while closeing via dragging #216
• Fix closing panel during internal scroll view bounce #218
• Remove workaround for tableView(_:didSelectRowAt:) issue #225
• Fix an unexpected layout update in the background on iOS13 #228

Thank you to @robbiet480 for your SwiftPM support!
Thank you to @cozzin, @heltisace and @Isuru-Nanayakkara for your issues and cooperations!