Apply minor changes to make AppData XML file friendlier to distributions

Some distros absolutely require a copyright header on this file.
Also, the license XML tag is not for the app, but for the xml file itself.
Finally, the image width and height parameters are entirely optional.
Keep the welcome dialog shown when clicking "Missing dependencies..."

Also prevent GTK3 from messing up the size of the missing dependencies dialog,
and remove an unused import in timeline/timeline.py

Fixes bug #711357
ui/startupwizard: Force ButtonBox items to have a homogeneous/uniform width

Fixes bug #682766
Releasing Pitivi 0.92 "Baby Steps"
Uploaded Ukranian
Make pitivi.appdata.xml more translation-friendly and pass appdata-validate

- Make individual description items translatable instead of _description
- Move the summary into the description to avoid length issues
- Remove "name" and "summary", rely on to pitivi.desktop instead