Introduce extensible `dtb:is-block-element` function
Don't print 'SKIPPING ...' messages
Improve text:span, text:a, text:p and text:h templates

* Use tunnel parameters `paragraph_style` and `text_style` directly
* Only wrap with text:span if needed (when a new language of style must be applied)
* text:span & text:h: support `sequence` and `apply-templates` parameters
* text:p & text:h: also wrap content in a text:span when `text_style` is set
Add dtb:lang utility function
Treat `dtb:poem` and `dtb:epigraph` similar to a `dtb:blockquote`
Support `byline`, `bridgehead`, `hd` and `covertitle`
Support `linegroup` and `line`
Support `cite`, `author`, `title`, `acronym`, `abbr`, `q`, `kbd` and `linenum`
Support `code` and `samp`

Can be either block or inline
Images: set default dpi value to 96
imggroup/caption: misc changes

* always put the caption above the image
* add prefix and suffix parameters
Page numbering: misc changes

* `pagenum_done` parameter to control where page numbers are inserted
* `insert-pagenum-after` template to skip a page number inside a
  paragraph, heading, list or table, and insert it at the end.
* SBS: add a prefix 'Seite '
Tables: handle @colspan and @rowspan
Treat annotations as notes
Alternative treatment of notes & annotations (SBS)

Instead of rendering notes as 'real' footnotes or endnotes, put them
inside round brackets and inline them after the noteref.
SBS: insert empty paragraph before imggroup/caption and a `:` after
Insert empty paragraphs before/after:

* headings
* lists
* blockquotes, epigraphs and poems
SBS: add 'Z' prefix before dtb:linenum
Fixup 143f62f: `text:empty-p` template
Support @xml:lang on brl:* elements