Remove bitrotted tab source artwork.
Add opposing-faction tab art for aircraft and navy,
Fix incorrect soviet tab image coordinates.
Enable correct tab images for helicopters and aircraft.
Fix bogus inconsistencies in the ra tab images:

- Soviet-normal tab glyphs were off by 1px.
- Allied-selected tabs had random semi-transparent pixels.
- Most tabs had a discoloured pixel near the left edge.
Merge pull request #5926 from pchote/ra-tabs

Fixes #5915