I completely forgot to rename the appstream file according to the new
ID. While doing so, I also make it a .in.in file, with initial
processing by the autotools. Indeed I need @GIMP_COMMAND@ to be replaced
by AC_CONFIG_FILES().
Finally I fix a badly closed XML tag (which reminds me I should always
test a commit, even when it's a simple non-C 1-liner change!).