Added `\setupinterlinespace` to `title`, `subtitle`, `date` and `author` elements.
Otherwise longer titles that run over multiple lines will look squashed as
`\tfd` etc. won't adapt the line spacing to the font size.
Fix `\startcslreferences`:
- The old version had a too large skip at the beginning of the reference list => fixed that.
- Change syntax to ConTeXt conventions
Previously they appeared centered at the top of the page;
now we put them centered at the bottom, unless the `pagenumbering`
variable is set (this gives users full control over page
number format and position,
https://wiki.contextgarden.net/Command/setuppagenumbering)
Define an start-stop-pair `cslreferences` to allow for hanging indents in the bibliography. Analogous to the cslreferences-environment in the default latex template. See here: https://github.com/jgm/pandoc-citeproc/issues/410
For this to work the context writer must be adapted.
* Let the user choose type of PDF/A generated with ConTeXt (closes#5608)
* Updated ConTeXt test documents for changes in tagging
* Updated color profile settings in accordance with ConTeXt wiki
* Made ICC profile and output intent for PDF/A customizable
* Read pdfa variable from meta (and updated manual)
The resulting PDF can be verified using the Apache PDFBox preflight app.
```
$ java -jar preflight-app-2.0.8.jar test.pdf
The file test.pdf is a valid PDF/A-1b file
```
Instructions on how to install the ICC profiles on ConTeXt standalone can be found in the wiki: <http://wiki.contextgarden.net/PDFX#ICC_profiles>.
If the ICC profiles are not available the log will contain messages like these
```
backend > profiles > profile specification 'sRGB.icc' loaded from '/usr/local/texlive/2017/texmf-dist/tex/context/colors/icc/context/colorprofiles.xml'
backend > profiles > error, couldn't locate profile 'srgb.icc'
backend > profiles > no default profile 'srgb.icc' for colorspace 'rgb'
backend > profiles > profile specification 'sRGB IEC61966-2.1' loaded from '/usr/local/texlive/2017/texmf-dist/tex/context/colors/icc/context/colorprofiles.xml'
backend > profiles > error, couldn't locate profile 'srgb.icc'
backend > profiles > invalid output intent 'sRGB IEC61966-2.1'
```
and the resulting PDF will not be valid PDF/A:
```
$ java -jar preflight-app-2.0.8.jar test.pdf
The file test.pdf is not a valid PDF/A-1b file, error(s) :
2.4.3 : Invalid Color space, The operator "g" can't be used without Color Profile on page 1
2.4.3 : Invalid Color space, The operator "G" can't be used without Color Profile on page 1
2.4.3 : Invalid Color space, /DeviceGray default for operator "TJ" can't be used without Color Profile on page 1
2.4.3 : Invalid Color space, /DeviceGray default for operator "TJ" can't be used without Color Profile on page 1
2.4.3 : Invalid Color space, The operator "g" can't be used without Color Profile on page 1
2.4.3 : Invalid Color space, The operator "G" can't be used without Color Profile on page 1
2.4.3 : Invalid Color space, /DeviceGray default for operator "TJ" can't be used without Color Profile on page 1
```
However, the PDF will still be generated and the "errors" shown in the log do not break anything.
