QML GridLayout (and RowLayout, ColumnLayout) don't support margins.

Consider the following common layout task:

ColumnLayout {
   RowLayout {
      // Label, text field, button
   }
   RowLayout {
      // Label, Spin box
   }
   RowLayout {
      // Button, button
   }
}

Currently it is virtually impossible to do all that layout correctly with nice looking margins.

1. `anchors.margin:` doesn't work when not using anchors.
2. `spacing:` only affects things inside the layout, it can't be used to add spacing around the layout.
3. You can add dummy spacer `Item`s but that only lets you add fake margins in the dimension of the layout (i.e. vertically for a ColumnLayout, horizontally for a RowLayout).

I'm not saying it's not impossible. It's just not nearly as easy as I've come to expect from Qt.

The solution is pretty easy, just create a `Layout.margins` similar to `anchor.margins`.