Keep default font-size on html, use rem accordingly

This makes sure the browser's minimal font-size does not affect the size of the
rem-unit. We used to have
  1rem = max(0.625*default-font-size, minimal-font-size).
Now we have
  1rem = default-font-size.
Of course this means that the use of rem needs fixing. This was done using the
following code:
<?php

while (!feof(STDIN)) {
	$l = fgets(STDIN);
	$l = preg_replace_callback(
		'/(-?[.0-9]+)(\s*rem)/',
		function ($matches) {
			return floatval($matches[1])*0.625 . $matches[2];
		},
		$l);
	fputs(STDOUT, $l);
}
Refactor get_(sidebar|content)_layout out

Formerly the body-class sidebar layout and sidebar.php sidebar layout could get
out of sync for attachments.
Refactor sidebar defaults to common location
Allow more types for the slider
Typoes, some more translatable pieces
Update POT

That is, run the following in languages:

find .. -iname *.php | xgettext -f - --language=php --from-code=utf8 \
--keyword=__ --keyword=_e --keyword=_n:1,2 --keyword=_x:1,2c \
--keyword=_ex:1,2c --keyword=esc_attr_e --keyword=esc_attr__ \
--add-comments=translators --output=- | msgmerge --update --width=80 \
--sort-by-file --backup=off catcheverest.pot -
Update .po-files from new .pot

That is, run the following:
  for f in *.po; do msgmerge --update --backup=off $f catcheverest.pot ; done
Update translations (insofar I can)

Also remove some untranslated stuff
Add Dutch Translation