Re #4224 Add "rename" item to Category context menu in companion. (#4360)

- Adds "rename" to the context menu for categories, it uses the
same code path as the default dblClick handler for the base class.

- Added a separator so the context menu looks more like the one
for models

- Some whitespace changes in mdichild.h, my editor truncates
whitespace on blank lines.

companion/src/modelslist.cpp

@@ -321,7 +321,7 @@ ModelsListWidget::ModelsListWidget(QWidget * parent):
 {
   setColumnWidth(0, 50);
   setColumnWidth(2, 100);
-  
+
   active_highlight_color = palette().color(QPalette::Active, QPalette::Highlight);
 }
 
@@ -337,7 +337,7 @@ void ModelsListWidget::mouseMoveEvent(QMouseEvent *event)
 {
   if (!(event->buttons() & Qt::LeftButton))
     return;
-  
+
   if ((event->pos() - dragStartPosition).manhattanLength() < QApplication::startDragDistance())
     return;