Make the qquickfontdialog support pixel size fonts in addition to point size

If you set the font dialog's selectedFont property to a font that is using pixelSize rather than pointSize. Then the list view for the font size in the non-native font dialog will be -1 (which is the internal QFont representation for pixel size fonts).

The following code can be used to verify the above information.

import QtQuick
import QtQuick.Controls
import QtQuick.Dialogs

ApplicationWindow {
    id: window

    width: 640
    height: 480
    visible: true

    FontDialog {
        id: dialog
        selectedFont: Qt.font({family: "Sans Serif", pixelSize: 15})
    }

    Button {
        anchors.centerIn: parent

        text: "Click me!"
        onClicked: dialog.open()
    }
}

Another issue related to this, is that if you select the header text in the Text Editor example in qtdeclarative/examples/quickcontrols2/texteditor
Then the font size will be incorrect, because the <h2> text will at some point be set to use a pixel size font. This might potentially be a bug, but I will need to investigate it further before I know for certain.