Doc: Adding "\keyword Signals and Slots" to "Signals & Slots" page.

-many articles link to the page using the "and" or the "&"
version, causing missing links.

Change-Id: I6447149befce169cfafff29164172290a7c15f0c
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/corelib/doc/src/objectmodel/signalsandslots.qdoc

@@ -28,6 +28,7 @@
 /*!
     \page signalsandslots.html
     \title Signals & Slots
+    \keyword Signals and Slots
     \ingroup qt-basic-concepts
     \brief An overview of Qt's signals and slots inter-object
     communication mechanism.