Doc: corrected broken links

Task-number: QTBUG-34749

Corrected link to drag and drop example.
Corrected link to {mandatory fields}
Created target referring to macros for defining plugins

Change-Id: I387a2d9bab428b2eacd8d371f08c72f42f7e2be2
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>

examples/widgets/doc/src/classwizard.qdoc

@@ -156,7 +156,7 @@
     Then we create the child widgets, create \l{Registering and Using
     Fields}{wizard fields} associated with them, and put them into
     layouts. The \c className field is created with an asterisk (\c
-    *) next to its name. This makes it a \l{mandatory field}, that
+    *) next to its name. This makes it a \l{mandatory fields}{mandatory field}, that
     is, a field that must be filled before the user can press the
     \uicontrol Next button (\uicontrol Continue on Mac OS X). The fields' values
     can be accessed from any other page using QWizardPage::field(),

examples/widgets/doc/src/echoplugin.qdoc

@@ -194,10 +194,10 @@
 
     \section1 Further reading and examples
 
-    You can find an overview of the macros needed to create plugins
-    \l{Macros for Defining Plugins}{here}.
+    The \l {qtplugin-defining-plugins}{Defining Plugins} page presents an overview of the macros needed to
+    create plugins.
 
-    We give an example of a plugin that extend Qt in the \l{Style
+    We give an example of a plugin that extends Qt in the \l{Style
     Plugin Example}{style plugin} example. The \l{Plug & Paint
     Example}{plug and paint} example shows how to create static
     plugins.

src/corelib/plugin/qplugin.qdoc

@@ -28,6 +28,7 @@
 /*!
     \headerfile <QtPlugin>
     \title Defining Plugins
+    \target qtplugin-defining-plugins
     \ingroup plugins
 
     \brief The <QtPlugin> header file defines macros for defining plugins.

src/gui/doc/src/dnd.qdoc

@@ -80,8 +80,8 @@
 
     The rest of the document focuses mainly on how to implement drag and drop
     in C++. For using drag and drop inside a Qt Quick scene, please read the
-    documentation for the Qt Quick \l{Drag}, \l{DragEvent} and \l{DropArea} items.
-    There is also an example \l{quick/draganddrop}{available}.
+    documentation for the Qt Quick \l{Drag}, \l{DragEvent}, and \l{DropArea} items,
+    as well as the \l {Qt Quick Examples - Drag and Drop}{Qt Quick Drag and Drop} examples.
 
     \section1 Dragging