Add a "max line length" field to customize QML auto-formatter

As of QtCreator 4.11.0, the max line length is hardcoded to 80 in QtCreator source code (qmljsreformatter.cpp, BestSplit class).

This value is way too small on HD screen displays and makes QML auto-formatter barely usable.

It would be a nice feature to have it customizable in Qt Quick options window (see below).