containments/panel: don't hang when the layout spacing is 0

childAtCoordinates() steps back by the layout's spacing until it finds
a widget. With a spacing of 0, which some third-party widgets set, the
point never moves, so dragging something over an empty part of the
panel froze plasmashell.

Always step back by at least 1 px.

BUG: 528228

containments/panel/LayoutManager.js

@@ -125,9 +125,9 @@ function childAtCoordinates(x, y) {
         if (root.isHorizontal) {
             // Only yields incorrect results for widgets smaller than the
             // row/column spacing, which is luckily fairly unrealistic
-            x -= layout.rowSpacing
+            x -= Math.max(1, layout.rowSpacing)
         } else {
-            y -= layout.columnSpacing
+            y -= Math.max(1, layout.columnSpacing)
         }
         if (x < 0 || y < 0) {
             return 0;