Context menu cleanups

Context menu cleanups

- Open the context menu on the mouse-press event, not on the
  mouse-release event.
- Provide an explicit position-information and don't use
  QCursor::pos(). This fixes the issue that opening a context-menu
  by the keyboard opens below the cursor.
- Provide different signals in the KItemListController for
  the different context-menu types (item vs. view vs. header).
- Implement turning on/off roles by the header-context-menu.