[customize-titlebar] Implement full Seamonkey support in titlebar customization

[xabolcs]

Refiled from #31 - the basic support.

@tonymec and I talked a little bit on mozillaZine about how to support Seamonkey.
It isn't trivial. SM has got two window types, at least. :)

Depends on #20 - wait for it's "output", and reuse from modules as much as possible.

Not sure about composer support.

This issue is not for:

• add support for message-read window, message-compose window in messenger.js
• add support for other extensions, bundled to Seamonkey: ChatZilla, Lightning, ...

[MikkCZ]

This is probably blocked by #246 now.

[xabolcs]

This was created for the legacy extension.
I vote for closing this, as this is now not applicable / outdated: the legacy case won't be fixed, and the Web Extension API case is covered by #246.