Always relayout when changing icon/text/font

Previously we would only relayout when creating/deleting the icon/text, but not when changing it.

E.g. the layout would not change size when text changed.

Probably fixes​ #13 (haven't actually tested KTrip or NeoChat)

There is one pre-existing bug caused when removing the icon, where the button actually gets a tiny amount larger. I have tracked this down to geometryChange being called with the wrong geometry (still seems to account for icon size) after emitting hasIconChanged, so the button uses a geometry larger than it should be using.

Tested with the following code:

ColumnLayout {
    QQC2.Button {
        id: button
        property bool shortMode: true
        property bool showIcon: false
        property bool useCourierFont: false
        property bool smallFontSize: true
        text: shortMode ? "Short text" : "Long text button"
        icon.name: showIcon ? "go-next" : ""
        font.family: useCourierFont ? "Courier" : "Helvetica"
        font.pointSize: smallFontSize ? 10 : 15
        Layout.alignment: Qt.AlignHCenter
    }
    QQC2.Button {
        text: "Toggle button text length"
        onClicked: button.shortMode = !button.shortMode
        Layout.alignment: Qt.AlignHCenter
    }
    QQC2.Button {
        text: "Toggle button icon"
        onClicked: button.showIcon = !button.showIcon
        Layout.alignment: Qt.AlignHCenter
    }
    QQC2.Button {
        text: "Toggle button font family"
        onClicked: button.useCourierFont = !button.useCourierFont
        Layout.alignment: Qt.AlignHCenter
    }
    QQC2.Button {
        text: "Toggle button font size"
        onClicked: button.smallFontSize = !button.smallFontSize
        Layout.alignment: Qt.AlignHCenter
    }
}