Rename metainfo file to appdata

gettext 0.19.7 gained support for translating appdata files, whereas
support for metainfo files was added in 0.20. These file formats are
effectively the same: as documented in the AppStream specification,
desktop applications can install files with the suffix .appdata.xml
rather than .metainfo.xml and these will still be handled correctly.

It is desirable for this project to support RHEL 7, which has gettext
0.19.8.1, so let's use the older filename.

As noted in the same section of the specification, appdata files were
previously installed to /usr/share/appdata rather than
/usr/share/metainfo, but the spec asserts that the newer metainfo path
works all the way back to RHEL 7, so we keep the newer installation
path.

Thanks to Shin-ichi TOYAMA for flagging this issue.

https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#spec-component-location

src/po/POTFILES.in.in

@@ -6,7 +6,7 @@ great.h
 im.c
 macos.h
 macos.m
-org.tuxpaint.Tuxpaint.metainfo.xml.in
+org.tuxpaint.Tuxpaint.appdata.xml.in
 shapes.h
 titles.h
 tools.h