Make iterating over Layer enum simple

Summary:
Having a function with a confusing comment, to iterate over the enum,
is mostly confusing. Make it boring by just moving the cast to the one
place that uses it.

Reviewers: #kwin, romangg, zzag

Reviewed By: #kwin, romangg, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23085

layers.cpp

@@ -530,10 +530,9 @@ ToplevelList Workspace::constrainedStackingOrder()
         layer[ l ].append(*it);
     }
     ToplevelList stacking;
-    for (Layer lay = FirstLayer;
-            lay < NumLayers;
-            ++lay)
-        stacking += layer[ lay ];
+    for (int lay = FirstLayer; lay < NumLayers; ++lay) {
+        stacking += layer[lay];
+    }
     // now keep transients above their mainwindows
     // TODO this could(?) use some optimization
     for (int i = stacking.size() - 1; i >= 0;) {

utils.h

@@ -90,12 +90,6 @@ enum Layer {
     NumLayers // number of layers, must be last
 };
 
-// yes, I know this is not 100% like standard operator++
-inline void operator++(Layer& lay)
-{
-    lay = static_cast< Layer >(lay + 1);
-}
-
 enum StrutArea {
     StrutAreaInvalid = 0, // Null
     StrutAreaTop     = 1 << 0,