Add launchable tag to appdata.xml

Please add the following line to the file src/xfce4-screenshooter.appdata.xml.in:

<launchable type="desktop-id">xfce4-screenshooter.desktop</launchable>

This is needed for appstream-generator to find the associated desktop file.

See also: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-launchable