Generic Project: cut common part of path in project explorer

This is necessary, if the project files on a
different directory than the source files. In this case, the project
explorer will show the full path e.g. in windows starting by C: for each
directory until the source files.

use-case:
some directory
|- project directory
   |- *.creator & *.files & ... generic project files
|- source directory
   |- some sub-directories and the source files

Fixes: QTCREATORBUG-19454
Change-Id: I95eee4afdc11adf4281220edc5f6ff29da41a3a5
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/genericprojectmanager/genericproject.h

@@ -66,6 +66,7 @@ private:
     QStringList processEntries(const QStringList &paths,
                                QHash<QString, QString> *map = nullptr) const;
 
+    static QString findCommonSourceRoot(const QStringList &list);
     void refreshCppCodeModel();
     void updateDeploymentData();
     void activeTargetWasChanged();