Provide a way to perform java script code in a readers page from external code

[tschob]

This PR:

• refactors the UIWebView extension to the subclass FolioReaderWebView
• Provides a way to let extneral code run java script in a FolioReaderPage

[hebertialmeida]

Why do you think is better to subclass the UIWebView instead of a extension, as it is not public to override?

[hebertialmeida]

Lets reduce the (webView as? FolioReaderWebView) cast, we can just cast with guard on beginning of the function.

guard let webView = webView as? FolioReaderWebView else { return }

Then just use webView as previously.

[tschob]

Yes this seems to be the better approach, I'll adjust it.

[hebertialmeida]

Same for casting here and so on...

[tschob]

I think it's better to use a subclass instead of an extension because:

• The added logic is only needed in one place and it only contains logic which is needed for the reader and useless for other use cases. With this needs it should normally be a subclass and not an extension.
• The extension effects all UIWebView instances in an app which uses the Folio Reader. If Folio Readers UIMenuController isn't overridden it's possible to select text in a app owned UIWebView instance and choose eg. "Highlight" (as the UIWebView extension says it can perform this action).
• Folio Readers UIWebView is less customisable as extension as everything which is added will effect all UIWebView instances in an app. So only things which don't break other UIWebView instances can be added. The old implementation breaks other instances.