Don't show null for empty files
Implement folded sections in viewer

Rationale viewing a long document on a small device isn't easy. Default
behavior now, hides all content under H1 sections. Clicking the heading
shows / hides the corresponding section.

This required to add javascript support to the webview, but no
user-provided code will be executed: no security problem introduced.
Move some utility functions to Filter abstract class.

This will help creating new filters much more easily.
Define a much more readable font size

This is handled in a new StyleFilter. This will be the place for all
default CSS changes in the future... and may be someday, some of the
values there will be configurable by the user.