Add <launchable/> tag to AppStream metadata

This still fixes an error as of appstream 1.0.2. Working on master:

$ appstreamcli validate --no-net --explain /com.github.dahenson.agenda.appdata.xml.in
I: com.github.dahenson.agenda:9: developer-name-tag-deprecated
   The toplevel `developer_name` element is deprecated. Please use the `name` element in a
   `developer` block instead.

I: com.github.dahenson.agenda:21: description-first-para-too-short
     A task manager to help you keep track of the tasks that matter most.
   The first `description/p` paragraph of this component might be too short (< 80 characters).
   Please consider starting with a longer paragraph to improve how the description looks like in
   software centers and to provide more detailed information on this component immediately in the
   first paragraph.

E: com.github.dahenson.agenda:~: desktop-app-launchable-missing
   This `desktop-application` component is missing a `desktop-id` launchable tag. This means that
   this application can not be launched and has no association with its desktop-entry file. It also
   means no icon data or category information from the desktop-entry file will be available, which
   will result in this application being ignored entirely.

I: com.github.dahenson.agenda:~: developer-info-missing
   This component contains no `developer` element with information about its author.

✘ Validation failed: errors: 1, infos: 3, pedantic: 2

dahenson / agenda

Add <launchable/> tag to AppStream metadata #148