Refactor medialist plugin

## OverView
SlidePlugin and MediaListPlugin has almost same data structure.
So change SlidePlugin to basement plugin for MediaListPlugin.

This pull-req is to change MediaListPlugin as extends SlidePlugin.

## Tasks
- [x] Add tests of data structure
- [x] Extends SlidePlugin
- [x] Override ajust~ methods
- [x] Add tests of view and data are combined
- [x] Resolve all tests

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@big2men 

Please Check My Code!!! :+1: