Include <release> in Flatpak appdata.xml

...fixing <https://github.com/flathub/org.libreoffice.LibreOffice/issues/13>
"No version information".  This is only minimal release information (cf.
<https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#
tag-releases>), but is what is available directly in the build without
additional manual input.  Lack of either a date or timestamp attribute causes a
failure "org.libreoffice.LibreOffice.desktop: AppData problem: attribute-missing
: <release> has no timestamp", so include the current date in ISO 8601 format to
silence that.

It is assumed that the LIBO_VERSION_* variables will only contain numeric
values, so don't need to be encoded when included in an XML attribute.

Change-Id: I442a01d5093ab2621897685c3bc1eeda4ee08fa9

solenv/bin/assemble-flatpak.sh

@@ -43,7 +43,7 @@ done
 ## doesn't show more than five screenshots anyway, so restrict to one each from
 ## the five libreoffice-*.appdata.xml: Writer, Calc, Impress, Draw, Base):
 mkdir /app/share/appdata
-cat <<\EOF >/app/share/appdata/org.libreoffice.LibreOffice.appdata.xml
+cat <<EOF >/app/share/appdata/org.libreoffice.LibreOffice.appdata.xml
 <?xml version="1.0" encoding="UTF-8"?>
 <component type="desktop">
  <id>org.libreoffice.LibreOffice.desktop</id>
@@ -98,6 +98,11 @@ cat <<\EOF >/app/share/appdata/org.libreoffice.LibreOffice.appdata.xml
   <kudo>ModernToolkit</kudo>
   <kudo>UserDocs</kudo>
  </kudos>
+ <releases>
+  <release
+    version="${LIBO_VERSION_MAJOR?}.${LIBO_VERSION_MINOR?}.${LIBO_VERSION_MICRO?}.${LIBO_VERSION_PATCH?}"
+    date="$(date +%Y-%m-%d)"/>
+ </releases>
 </component>
 EOF