Also removed TokenUnderCursor as it's functionality is in the token cache.

Reviewed-by: ckamm

This reverts commit c2393df0.

This should speed things up a bit, because before, the line was tokenized at
least 3 times.

Done with: erikv

Several special cases that are handled by the CPPEditor did not take
into account code that was using tab characters.

Task-number: QTCREATORBUG-292
Reviewed-by: Roberto Raggi

Reviewed-by: Thorbjørn Lindeijer <thorbjorn.lindeijer@nokia.com>

Added BackwardsScanner::mid(tokenIndex). Mid() returns a substring starting from the position of the given token.