User Manual: Ensure the |lang| and |app| substitutions work everywhere

Also ensure that *all* .. include directives have their substitutions
properly processed, in case I add any more in the future.
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E-book viewer: Fix page turning by clicking in the margins in full screen mode (when displaying more than one page per screen) not working because of a Qt 5 behavior change.
Start work on completion popup for editor
Implement key based scrolling in the popup widget
Edit Book: More descriptive error message when saving fails because of a bad umask
A spot of refactoring