Scale bar metric and imperial units for Settings

Nobody has claimed this yet.

Imperial units may be useful for users in U.S., Myanmar, .. A combo box can be added to settings to choose metric or imerial units.
for main.qml:
```
// Scale bar units
function scalebar_settings(){
if(scalebar_combo.currentIndex === 0){
return QgsQuick.QgsUnitTypes.MetricSystem
}else{
return QgsQuick.QgsUnitTypes.ImperialSystem
}
}
ScaleBar {
id: scaleBar
anchors.bottomMargin: 10
mapSettings: mapCanvas.mapSettings
preferredWidth: Math.min(window.width, 180 * QgsQuick.Utils.dp)
z: zToolkits
systemOfMeasurement: Util.scalebar_settings()
anchors.horizontalCenter: parent.horizontalCenter
}
```
for settings:
```
RowLayout{
Text{
text:qsTr("Scale bar unit: ")
}
QQC2.ComboBox {
id: scalebar_combo
implicitWidth: 200
currentIndex: 0
model: ListModel {
ListElement { text: qsTr("Metric") }
ListElement { text: qsTr("Imperial") }
}
}
}
```
It would be easy to create combo box or radio box dialog with material design. It is just an idea :)

Start with main.qml and the settings UI described in the issue, then trace how the existing scale bar and settings values are connected. Add a Metric/Imperial choice that controls the scale bar's unit system, and verify that changing the choice visibly changes the scale bar units.

Written by the indexing model from the issue text.