Doc: Remove irrelevant sections from "Signals & Slots"

- This article is not the right place to describe the low-level
  mechanisms of moc and qmake, or to discuss QMetaObject features that
  are unrelated to signals and slots.
- Most users never need to run moc directly.
- The current content only mentions qmake for moc automation, but CMake,
  QBS, and the Visual Studio Add-In can also do that.

In light of the above 3 points, let's simply link to the "Meta-Object
System" article for those who are interested in the behind-the-scenes
details.

Most of the content deleted by this patch are already discussed in
detail in the articles "The Meta-Object System" and "Using the Meta-
Object Compiler (moc)" (the former links to the latter). The exception
is Snippet 5 -- this is deleted without replacement because
qobject_cast() is a much safer alternative to QMetaObject::inherits()
with static_cast(), so we should encourage the former.

Change-Id: I638c888cedfcdfb818747edeb806213ebd54dfb6
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/doc/snippets/signalsandslots/signalsandslots.cpp

@@ -38,8 +38,6 @@
 **
 ****************************************************************************/
 
-#include <QAbstractButton>
-
 #include "signalsandslots.h"
 
 //! [0]
@@ -66,19 +64,4 @@ int main()
 //! [3] //! [4]
     b.setValue(48);     // a.value() == 12, b.value() == 48
 //! [4]
-
-
-    QWidget *widget = reinterpret_cast<QWidget *>(new QObject(0));
-//! [5]
-    if (widget->inherits("QAbstractButton")) {
-        QAbstractButton *button = static_cast<QAbstractButton *>(widget);
-        button->toggle();
-//! [5] //! [6]
-    }
-//! [6]
-
-//! [7]
-    if (QAbstractButton *button = qobject_cast<QAbstractButton *>(widget))
-        button->toggle();
-//! [7]
 }

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

@@ -36,7 +36,8 @@
     Signals and slots are used for communication between objects. The
     signals and slots mechanism is a central feature of Qt and
     probably the part that differs most from the features provided by
-    other frameworks.
+    other frameworks. Signals and slots are made possible by Qt's
+    \l{The Meta-Object System}{meta-object system}.
 
     \tableofcontents
 
@@ -182,17 +183,6 @@
     \l{Using a Designer UI File in Your Application#Automatic Connections}
     {automatic connections} feature.
 
-    \section1 Building the Example
-
-    The C++ preprocessor changes or removes the \c{signals},
-    \c{slots}, and \c{emit} keywords so that the compiler is
-    presented with standard C++.
-
-    By running the \l moc on class definitions that contain signals
-    or slots, a C++ source file is produced which should be compiled
-    and linked with the other object files for the application. If
-    you use \l qmake, the makefile rules to automatically invoke \c
-    moc will be added to your project's makefile.
 
     \section1 Signals
 
@@ -267,27 +257,6 @@
     alongside a Qt-based application. To solve this problem, \c
     #undef the offending preprocessor symbol.
 
-    \section1 Meta-Object Information
-
-    The meta-object compiler (\l moc) parses the class declaration in
-    a C++ file and generates C++ code that initializes the
-    meta-object. The meta-object contains the names of all the signal
-    and slot members, as well as pointers to these functions.
-
-    The meta-object contains additional information such as the
-    object's \l{QMetaObject::className()}{class name}. You can
-    also check if an object \l{QObject::inherits()}{inherits}
-    a specific class, for example:
-
-    \snippet signalsandslots/signalsandslots.cpp 5
-    \snippet signalsandslots/signalsandslots.cpp 6
-
-    The meta-object information is also used by qobject_cast<T>(), which
-    is similar to QObject::inherits() but is less error-prone:
-
-    \snippet signalsandslots/signalsandslots.cpp 7
-
-    See \l{Meta-Object System} for more information.
 
     \section1 A Real Example