Read from install-wide settings for registering documentation.

Installers can write a semicolon separated list of files and directories
to the install-wide settings for the key "Help/InstalledDocumentation".
Qt Creator will look for *.qch files in directories mentioned there
(*not* traversing subdirectories).

Change-Id: I0f9b92caee3e2c72b3876573b9a050f7aea24a1a
Reviewed-by: Karsten Heimrich <karsten.heimrich@nokia.com>
Reviewed-by: Tim Jenssen <tim.jenssen@nokia.com>
Reviewed-by: Niels Weber <niels.2.weber@nokia.com>

src/plugins/coreplugin/helpmanager.h

@@ -93,7 +93,6 @@ signals:
 
 private slots:
     void setupHelpManager();
-    void collectionFileModified();
 
 private:
     void verifyDocumenation();