Utils: Pass dialog parent to Utils::* file dialog

Amends 3edc5673b5.

Turns out quite a few potential uses have other parents than
ICore::dialogParent().

Use a nullptr parent to mean ICore::dialogParent() to keep the
caller side simple.

Change-Id: Icfe1daafd710ae273d286679e0c8e2a3a27da552
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/projectexplorer/projectwindow.cpp

@@ -755,8 +755,8 @@ public:
         QTC_ASSERT(projectImporter, return);
 
         FilePath importDir =
-                Utils::FileUtils::getExistingDirectory(ProjectWindow::tr("Import Directory"),
-                                                       project->projectDirectory());
+                FileUtils::getExistingDirectory(nullptr, ProjectWindow::tr("Import Directory"),
+                                                project->projectDirectory());
 
         Target *lastTarget = nullptr;
         BuildConfiguration *lastBc = nullptr;