EESchema remove global variable and fix text object change type undo/redo.

* Move undo item copy global variable into schematic editor frame object
  member variable.
* Add helper methods for accessing the undo item copy member variable.
* Fix undetected bug when changing a text type.
* Added an exchange command to the undo/redo base class for handling undoing
  a changed item type which cannot be undone by swapping out the variables.
* Revert change to common/hotkeys_basic.cpp that broke hot key behavior.
* Lots of coding policy changes while making the changes above.

eeschema/busentry.cpp

@@ -86,12 +86,9 @@ void SCH_EDIT_FRAME::StartMoveBusEntry( SCH_BUS_ENTRY* BusEntry, wxDC* DC )
         return;
 
     if( !BusEntry->IsNew() )    // not already in edit, save shape
-    {
-        delete g_ItemToUndoCopy;
-        g_ItemToUndoCopy = BusEntry->Clone();
-    }
+        SetUndoItem( BusEntry );
 
-    BusEntry->m_Flags |= IS_MOVED;
+    BusEntry->SetFlags( IS_MOVED );
 
     ItemInitialPosition = BusEntry->m_Pos;