Doc: use cross-linking to Qt module documentation

Replace links to "\l{http://qt-project.org/doc/"
with links to topic titles. Remove links where QDoc
creates them automatically based on the .index files.
Add the .index files for the modules that are discussed
in the Qt Creator Manual.

Note: the links will only be generated if the Qt used to
build the docs contains the linked modules and documentation
(.index file) has been generated for them.

Change-Id: Ibe624bf3773e7c854c03ebba4db406be0b4a7b90
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

doc/src/howto/creator-cli.qdoc

@@ -161,11 +161,11 @@
 
     \section1 Using Custom Styles
 
-    \QC is a \l{http://qt-project.org/doc/qt-5.0/qtwidgets/qapplication.html#QApplication}
+    \QC is a \l{QApplication}
     {Qt application}, and therefore, it accepts the command line options
     that all Qt applications accept. For example, you can use the \c {-style} and
     \c {-stylesheet} options to apply custom styles and
-    \l{http://qt-project.org/doc/qt-5.0/qtwidgets/stylesheet.html}{stylesheets}.
+    \l{QApplication#stylesheet}{stylesheets}.
     The styling is only applied during the current session.
 
     Exercise caution when applying styles, as overriding the existing styling

doc/src/howto/creator-external-tools.qdoc

@@ -55,9 +55,7 @@
     specify other command line arguments for the tools, select \gui {Tools >
     External > Configure}.
 
-    For more information about Qt Linguist, see
-    \l{http://qt-project.org/doc/qt-5.0/qtlinguist/qtlinguist-index.html}
-    {Qt Linguist Manual}.
+    For more information about Qt Linguist, see \l{Qt Linguist Manual}.
 
     \section1 Previewing QML Files
 

doc/src/howto/creator-help.qdoc

@@ -177,9 +177,7 @@
         \li Create a .qch file from your documentation.
 
             For information on how to prepare your documentation and create a
-            .qch file, see
-            \l{http://qt-project.org/doc/qt-5.0/qthelp/qthelp-framework.html}
-            {The Qt Help Framework}.
+            .qch file, see \l{The Qt Help Framework}.
 
         \li To add the .qch file to \QC, select \gui Tools > \gui Options >
             \gui Help > \gui Documentation > \gui Add.