Javascript: Stricter 'use strict' completion

Only activate at the beginning of a function or file.
Fixes #246
Javascript: Use GetContainingLine()

Should be faster than querying Lines.
Also use correct TextBuffer in CompletionSources.
Merge pull request #259 from SLaks/use-strict-fix

Stricter 'use strict' completion
Cleanup: Removed unnecessary code.
Merge: Mad's master branch with resolved conflits.