It used to be genrated and installed by the old custom build system,
but apparently got lost in switching to meson.
Rename to .in.in instead of .template.in, since i18n otherwise gets
utterly confused failing to recognise the file type and both erroring
out with "no ITS rules found" during merge_file and failing to extract
any translatable strings.
Also drop underscores which were previously processed by intltool.
This doesn't yet automatically replace the release version and date
in the appdata file, but neither did the old build.
Note: while this slightly improves the situation, meson’s aegisub-pot
target is still missing many strings. make_pot.sh should be used.
