Remove 'Xngine' strings.
Fix catastrophic indentation issues in trueskill.lua

Closes #224
Fix custom backgrounds in skirmish/LAN.

Fixes #225
Disable collision on sinking air wrecks

Air wrecks were colliding will other units, sometimes causing them to get
stuck in naval units.

This resulted in scouts having intel so long as they didn't sink to
sea-bottom and get wrecked

Fixes #194, closes #222
Update in-lobby changelog
Fix typo in changelog
Trivial checkbox logic cleanup.
Correctly spell 'developers'.
Clarify some tooltip wordings.
Use a RadioButton control for background selection.
Prevent options dialog components from appearing in front of exit dialog.
Remove theme switching buttons. The theme switching does not exist yet: create a feature branch for this work and add the buttons there. Don't ship buttons that do nothing!
Localise various lobby strings.
Give profile values more descriptive names.
Don't convert a number to a string and back again when setting font size.
Default system messages to enabled.
Deduplicate system message logic.
Don't use a list to check for oddness.
Simplify manual autoteam logic.
Trivial path segment caching cleanup.