Fix crash when :xccmd jumpToNextCounterPart is executed.
The cause was that DVTFoldingTextStorage's decreaseUsingFoldedRanges method.
We have to call the method with paired to increaseUsingFoldedRanges.
So "increase..." must be called before "decrease..."
They have been called in XVimWindow's handleKeyStroke method.
But if the view is switched during handling a keystroke the "decrease..." method alone is called to the new view.
It leads crash. Now "increase/decrease" are called inside each view. (xvim_string has implemented)
We must use xvim_string to access to a string in a text view.