Replace WebView by a Text or Source view

[cgueret]

Currently the app uses a WebView widget to display the preview of the manuscript. This has the inconvenient to make the app harder to publish on systems which do not package it easily. It also may not be flexible to extend the application towards publishing other formats (Markdown, text, etc). This issue will be used to investigate replacing this widget by a text view, or maybe a source view if that would be beneficial