various: consolidated dir exists+mkpath into one function

app/dialog/export/export.cpp

@@ -313,7 +313,7 @@ void ExportDialog::StartExport()
 
   // If the directory does not exist, try to create it
   QDir dest_dir(file_info.path());
-  if (!dest_dir.exists() && !dest_dir.mkpath(QStringLiteral("."))) {
+  if (!FileFunctions::DirectoryIsValid(dest_dir)) {
     QtUtils::MessageBox(this, QMessageBox::Critical, tr("Failed to create output directory"),
                         tr("The intended output directory doesn't exist and Olive couldn't create it. "
                                          "Please choose a different filename."));

app/dialog/preferences/tabs/preferencesdisktab.cpp

@@ -96,7 +96,7 @@ bool PreferencesDiskTab::Validate()
     }
 
     // Check validity of the new path
-    if (!FileFunctions::DirectoryIsValid(disk_cache_location_->text(), true)) {
+    if (!FileFunctions::DirectoryIsValid(disk_cache_location_->text())) {
       QMessageBox::critical(this,
                             tr("Disk Cache"),
                             tr("Failed to set disk cache location. Access was denied."));

app/dialog/projectproperties/projectproperties.cpp

@@ -186,7 +186,7 @@ void ProjectPropertiesDialog::accept()
 
 bool ProjectPropertiesDialog::VerifyPathAndWarnIfBad(const QString &path)
 {
-  if (!FileFunctions::DirectoryIsValid(path, true)) {
+  if (!FileFunctions::DirectoryIsValid(path)) {
     QMessageBox mb(this);
     mb.setWindowModality(Qt::WindowModal);
     mb.setIcon(QMessageBox::Critical);