fix "preview starts always with mainFile and keeps root item size" in a proper way

This brings a problem when changing the current file to another different sized file then the mainFile. It keeps the first initialized size of the root item.

in qtcreator\src\plugins\qmlpreview\qmlpreviewconnectionmanager.cpp

there is code:

        if (!m_fileClassifier(changedFile)) {
            emit restart();
            return;
        }

maybe doing this for current file change will be the solution

also, this brings a problem when a component is used in the main.qml file is broken every other component can not be previewed