Adding self hosted update feature

Using the classes from [Plugin Update Checker](https://github.com/YahnisElsts/plugin-update-checker). The update check is done against the version number set in the file `/inc/update.json`. I think the debug files from PUC could be removed, but I've never looked into that.

Other modifications:
- added donate link in the plugin row meta
- relative paths to images in readme.md
- moved screenshots to `/images`
- CSS adjustment for the accordion indicators
- jQuery to fill the screen when opening the dialog popup