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/project.cpp

@@ -721,6 +721,7 @@ Utils::FilePath Project::projectDirectory(const Utils::FilePath &top)
 void Project::changeRootProjectDirectory()
 {
     Utils::FilePath rootPath = Utils::FileUtils::getExistingDirectory(
+          nullptr,
           tr("Select the Root Directory"),
           rootProjectDirectory(),
           QFileDialog::ShowDirsOnly | QFileDialog::DontResolveSymlinks);