Fix remaining problems preventing unity builds to work

* Avoid multiple definitions of `centerObj()`; this was very bad anyway
* Include `c++utilities/tests/testutils.h` upfront so the formatting
  operators are included before CppUnit

testhelper/helper.h

@@ -4,6 +4,7 @@
 #include "./global.h"
 
 #include <c++utilities/conversion/stringbuilder.h>
+#include <c++utilities/tests/testutils.h>
 
 #ifndef SYNCTHINGTESTHELPER_FOR_CLI
 #include <cppunit/extensions/HelperMacros.h>

tray/gui/devbuttonsitemdelegate.cpp

@@ -1,5 +1,7 @@
 #include "./devbuttonsitemdelegate.h"
 
+#include "./helper.h"
+
 #include <syncthingconnector/syncthingconnection.h>
 #include <syncthingmodel/syncthingdevicemodel.h>
 #include <syncthingmodel/syncthingicons.h>
@@ -20,11 +22,6 @@ using namespace Data;
 
 namespace QtGui {
 
-inline int centerObj(int avail, int size)
-{
-    return (avail - size) / 2;
-}
-
 DevButtonsItemDelegate::DevButtonsItemDelegate(QObject *parent)
     : QStyledItemDelegate(parent)
 {

tray/gui/dirbuttonsitemdelegate.cpp

@@ -1,5 +1,7 @@
 #include "./dirbuttonsitemdelegate.h"
 
+#include "./helper.h"
+
 #include <syncthingmodel/syncthingdirectorymodel.h>
 #include <syncthingmodel/syncthingicons.h>
 
@@ -19,11 +21,6 @@ using namespace Data;
 
 namespace QtGui {
 
-inline int centerObj(int avail, int size)
-{
-    return (avail - size) / 2;
-}
-
 DirButtonsItemDelegate::DirButtonsItemDelegate(QObject *parent)
     : QStyledItemDelegate(parent)
 {

tray/gui/downloaditemdelegate.cpp

@@ -1,5 +1,7 @@
 #include "./downloaditemdelegate.h"
 
+#include "./helper.h"
+
 #include <syncthingmodel/syncthingdownloadmodel.h>
 #include <syncthingmodel/syncthingicons.h>
 
@@ -23,11 +25,6 @@ using namespace Data;
 
 namespace QtGui {
 
-inline int centerObj(int avail, int size)
-{
-    return (avail - size) / 2;
-}
-
 DownloadItemDelegate::DownloadItemDelegate(QObject *parent)
     : QStyledItemDelegate(parent)
 {

tray/gui/helper.h

@@ -103,6 +103,11 @@ template <typename ViewType, typename ActionType> inline auto triggerActionForSe
     };
 }
 
+inline int centerObj(int avail, int size)
+{
+    return (avail - size) / 2;
+}
+
 } // namespace QtGui
 
 #endif // TRAY_GUI_HELPER_H