Set default extensions for most save files

In most of these cases, the open file dialog won't even display a file
without the proper extension, so this helps ensure the user doesn't
accidentally misplace their files. The one exception is *.rdc, which
could be found without the extension, but could not be opened.

qrenderdoc/Windows/MainWindow.cpp

@@ -566,8 +566,8 @@ QString MainWindow::GetSavePath()
       dir = m_LastSaveCapturePath;
   }
 
-  QString filename =
-      RDDialog::getSaveFileName(this, tr("Save Capture As"), dir, tr("Capture Files (*.rdc)"));
+  QString filename = RDDialog::getSaveFileName(this, tr("Save Capture As"), dir,
+                                               tr("Capture Files (*.rdc)"), lit("rdc"));
 
   if(!filename.isEmpty())
   {