start timer with invokeMethod

rectSaveTimer is used to event compress the save and sync of the
configuration file that is used as a cache of the rectangles of
svg items (in order to avoid svg renderers creation when possible)
there is exactly one situation where it seems to not work: during
creation of svgitems/framesvgitems, insertintorectcache ends up
being called by another thread than the Theme's qobject thread,
and that is not allowed.

with invokeMethod the timer will be started by the correct thread
REVIEW:122915

Change-Id: I3ff4a4ebd1dfdba4b9b830fe6c08feece19b903e

src/plasma/theme.cpp

@@ -28,6 +28,7 @@
 #include <QPair>
 #include <QStringBuilder>
 #include <QTimer>
+#include <QThread>
 
 #include "config-plasma.h"
 
@@ -417,7 +418,7 @@ void Theme::insertIntoRectsCache(const QString &image, const QString &element, c
         }
     }
 
-    d->rectSaveTimer->start();
+    QMetaObject::invokeMethod(d->rectSaveTimer, "start");
 }
 
 void Theme::invalidateRectsCache(const QString &image)