BC: Move builddirectory handling into BC itself

Use setBuildDirectory() in the different BuildConfigurations instead
of reimplementing that over and over again.

Change-Id: Ic355fdb4624c71667ce470b3e2865c9a8722ef09
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/valgrind/memchecktool.cpp

@@ -145,7 +145,7 @@ bool MemcheckErrorFilterProxyModel::filterAcceptsRow(int sourceRow, const QModel
             validFolders << project->projectDirectory();
             foreach (Target *target, project->targets()) {
                 foreach (BuildConfiguration *config, target->buildConfigurations())
-                    validFolders << config->buildDirectory();
+                    validFolders << config->buildDirectory().toString();
             }
         }