gschema fixes

[jbicha]

When building Errands for Debian, I got this build test error:

==================================== 2/2 =====================================
test:         Validate schema file
start time:   15:12:49
duration:     0.01s
result:       exit status 1
command:      MALLOC_PERTURB_=228 /usr/bin/glib-compile-schemas --strict --dry-run /<<PKGBUILDDIR>>/data
----------------------------------- stderr -----------------------------------
/<<PKGBUILDDIR>>/data/io.github.mrvladus.List.gschema.xml:4:1  Error on line 4 char 1:
 A path, if given, must begin and end with a slash.  --strict was specified; exiting.

I was able to fix it by using a .in suffix for the gschema file that hasn't yet been configured (in other words, its PATH wasn't yet valid).

I happened to notice that the gschema file wasn't listed in po/POTFILES either so there's a second commit for that.

[mrvladus]

Hello! I only have .desktop file translatable on purpose :)

[jbicha]

Ok, I'll make a separate merge proposal for translations