Doc: edit information about i18n

Make "Internationlization with Qt" about the classes that
support i18n and move all other information to "Writing Source
Code for Translation".

Combine with information from the Qt Linguist Manual and remove
redundancy.

Change-Id: If7f4d7702ca7fcffcca262f33494867a028449d0
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

doc/src/externalsites/qt-webpages.qdoc

@@ -100,3 +100,8 @@
     \externalpage http://qt-project.org/doc/qt-4.8/qtquick.html
     \title Qt Quick 1 Reference Documentation
 */
+
+/*!
+    \externalpage http://qt-project.org/wiki/Qt-Localization
+    \title external: Translating Qt Into Other Languages
+*/

doc/src/snippets/code/doc_src_i18n.cpp

@@ -173,3 +173,19 @@ void MyWidget::changeEvent(QEvent *event)
         QWidget::changeEvent(event);
 }
 //! [12]
+
+
+//! [13]
+void some_global_function(LoginWidget *logwid)
+{
+    QLabel *label = new QLabel(
+            LoginWidget::tr("Password:"), logwid);
+}
+
+void same_global_function(LoginWidget *logwid)
+{
+    QLabel *label = new QLabel(
+            qApp->translate("LoginWidget", "Password:"),
+            logwid);
+}
+//! [13]