Enhance xdg files

[scx]

Enhance desktop file

1. Update Categories https://standards.freedesktop.org/menu-spec/latest/apa.html https://standards.freedesktop.org/menu-spec/latest/apas02.html

2. Change GenericName GenericName should be a generic name of the application, for example "Web Browser". The value should not be redundant with the values of Name and Comment. https://developer.gnome.org/desktop-entry-spec/#recognized-keys

3. Add StartupWMClass and StartupNotify http://www.freedesktop.org/Standards/startup-notification-spec

Enhance AppData file

1. Update release notes

   • Fix date for 0.5.0
   • Add info about 0.5.1

2. Add screenshot captions

3. Add kudos

4. Add launchable tag

5. Fix indents

See also: https://github.com/flathub/flathub/wiki/AppData-Guidelines https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-launchable https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-releases https://gitlab.gnome.org/GNOME/gnome-software/blob/master/doc/kudos.md#kudos-used-in-software

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This PR is related to submitting my flatpak package to the Flathub repository. https://github.com/parnold-x/nasc/issues/105 https://github.com/flathub/flathub/pull/867

See also: https://github.com/scx/nasc/pull/2

[scx]

Before reviewing, please read this: https://github.com/parnold-x/nasc/pull/110#issue-253611449