Fixes#20510.
The application now appears in system menus, and Konqueror now suggests
opening .xoj files with Xournal. Other file browsers should as well.
Previously we relied on an environment variable POPPLER_DATADIR which
practically noone used and everyone was expected to set. This is a good
candidate for a feature option because noone really _noticed_ that this data is
not available. Disabled by default because of this and size of the data (22M).
In line with the Nixpkgs manual.
A mechanical change, done with this command:
find pkgs -name "*.nix" | \
while read f; do \
sed -e 's/description\s*=\s*"\([a-z]\)/description = "\u\1/' -i "$f"; \
done
I manually skipped some:
* Descriptions starting with an abbreviation, a user name or package name
* Frequently generated expressions (haskell-packages.nix)
