Task-number: QTCREATORBUG-1776

Done-with: Erik Verbruggen

Previously, it would still fail if a forward declaration and declaration
for a class were in the same file.

Task-number: QTCREATORBUG-20

columnNumber() relies on text wrapping, and we're only interested in the
position of the cursor in the block.

If it encounters a forward declaration, it tries to find the class
declaration globally now.

Task-number: QTCREATORBUG-20

Due to using QTextCursor::columnNumber() instead of
QTextCursor::positionInBlock(), a lot of code would not work correctly
when used with wrapped lines.

In addition, there was an issue with columnNumber() returning 0 right
after inserting a character before the last character of a line.

Reviewed-by: mae

Reviewed-by: ogoffart

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.

Task-number: QTCREATORBUG-1501
Reviewed-by: Roberto Raggi