Blender Git Loki

FreeDesktop: Add Release information to AppStream

This commit improves Blender's AppStream metadata with minor punctuation and grammar improvements to the description, and a new `<release>` tag for the current release.

The `<release>` tag benefits Blender in the following ways:
- Flathub users get to see the actual release number instead of "stable"
- Make it easy for all packagers to include changelogs, so Blender's users gets to see what's new in 2.79
- Blender gets featured in GNOME software to reward it for having Release information in its AppStream metadata (KDE Discover [[ https://bugs.kde.org/show_bug.cgi?id=389509 | plans do do the same thing ]])

If the prior objection to adding release data to the AppStream metadata was the ongoing maintenance burden, I volunteer to take it on for subsequent Blender releases.

Author: @ngraham

Differential Revision: https://developer.blender.org/D3065