effects/desktopgrid: Replace PlasmaCore.Units.largeSpacing with gridUnit

It's broken and equal to a grid unit, all existing usages must ported
one way or another. Using `smallSpacing * 2` (the intended value of
largeSpacing) results in a gap that is too narrow to be consistent when
scaled down between multiple desktops.

src/effects/desktopgrid/qml/main.qml

@@ -188,8 +188,8 @@ Rectangle {
 
         width: parent.width * columns
         height: parent.height * rows
-        rowSpacing: PlasmaCore.Units.largeSpacing
-        columnSpacing: PlasmaCore.Units.largeSpacing
+        rowSpacing: PlasmaCore.Units.gridUnit
+        columnSpacing: PlasmaCore.Units.gridUnit
         rows: container.effect.gridRows
         columns: container.effect.gridColumns
         transformOrigin: Item.TopLeft